Form of Letter of Transmittal

THE OFFER AND WITHDRAWAL RIGHTS WILL EXPIRE AT 12:00 MIDNIGHT, NEW YORK CITY TIME, ON MONDAY, JUNE 29, 2009, UNLESS THE OFFER IS EXTENDED.

DELIVERY OF THIS LETTER OF TRANSMITTAL TO AN ADDRESS OTHER THAN AS SET FORTH ABOVE OR TRANSMISSION OF INSTRUCTIONS VIA FACSIMILE TO A NUMBER OTHER THAN AS SET FORTH ABOVE WILL NOT CONSTITUTE A VALID DELIVERY. YOU MUST SIGN THIS LETTER OF TRANSMITTAL WHERE INDICATED BELOW AND COMPLETE EITHER THE IRS FORM W-9 ACCOMPANYING THIS LETTER OF TRANSMITTAL OR AN APPLICABLE IRS FORM W-8. SEE INSTRUCTION 9 BELOW.

PLEASE READ THE INSTRUCTIONS ACCOMPANYING THIS LETTER OF TRANSMITTAL CAREFULLY BEFORE COMPLETING THIS LETTER OF TRANSMITTAL.

IF YOU WOULD LIKE ADDITIONAL COPIES OF THIS LETTER OF TRANSMITTAL OR ANY OF THE OTHER OFFERING DOCUMENTS, YOU SHOULD CONTACT THE INFORMATION AGENT, MORROW & CO., LLC, AT (203) 658-9400 OR (800) 662-5200.

You have received this Letter of Transmittal in connection with the offer of Envoy Merger Corporation, a Delaware corporation (“Purchaser”) and a wholly owned subsidiary of EMC Corporation, a Massachusetts corporation (“EMC”), to purchase all outstanding shares of Data Domain, Inc., a Delaware corporation (“Data Domain”), at a price of $30.00 per Share (as defined below) net to the tendering stockholder in cash, without interest thereon and subject to reduction for any applicable withholding taxes, as described in the Offer to Purchase, dated June 2, 2009.

You should use this Letter of Transmittal to deliver to the Depositary shares of common stock, par value $0.0001 per share, of Data Domain (“Shares”) represented by stock certificates for tender. If you are delivering your Shares by book-entry transfer to an account maintained by the Depositary at The Depository Trust Company (“DTC”), you may use this Letter of Transmittal or you may use an Agent’s Message (as defined in Instruction 2 below). In this document, stockholders who deliver certificates representing their Shares are referred to as “Certificate Stockholders”. Stockholders who deliver their Shares through book-entry transfer are referred to as “Book-Entry Stockholders”.

If certificates for your Shares are not immediately available or you cannot deliver your certificates and all other required documents to the Depositary on or prior to the Expiration Date (as defined in Section 1 of the Offer to Purchase), or you cannot comply with the book-entry transfer procedures on a timely basis, you may nevertheless tender your Shares according to the guaranteed delivery procedures set forth in Section 3 of the Offer to Purchase. See Instruction 2. Delivery of documents to DTC will not constitute delivery to the Depositary.

The undersigned hereby tenders to Envoy Merger Corporation, a Delaware corporation (“Purchaser”) and a wholly owned subsidiary of EMC Corporation, a Massachusetts corporation (“EMC”), the above-described shares of common stock, par value $0.0001 per share (“Shares”), of Data Domain, Inc., a Delaware corporation (“Data Domain”), pursuant to the Offer to Purchase, dated June 2, 2009 (the “Offer to Purchase”), at a price of $30.00 per Share, net to the seller in cash, without interest thereon and subject to reduction for any applicable withholding taxes, on the terms and subject to the conditions set forth in the Offer to Purchase, receipt of which is hereby acknowledged, and this Letter of Transmittal (which, together with the Offer to Purchase, as each may be amended or supplemented from time to time, collectively constitute the “Offer”). The undersigned understands that Purchaser reserves the right to transfer or assign, from time to time, in whole or in part, to one or more of its affiliates, the right to purchase the Shares tendered herewith.

On the terms and subject to the conditions of the Offer (including, if the Offer is extended or amended, the terms and conditions of such extension or amendment), subject to, and effective upon, acceptance for payment and payment for the Shares validly tendered herewith in accordance with the terms of the Offer, the undersigned hereby sells, assigns and transfers to, or upon the order of, Purchaser, all right, title and interest in and to all of the Shares being tendered hereby and any and all cash dividends, distributions, rights, other Shares or other securities issued or issuable in respect of such Shares on or after June 1, 2009 (collectively, “Distributions”). In addition, the undersigned hereby irrevocably appoints BNY Mellon Shareowner Services (the “Depositary”) the true and lawful agent and attorney-in-fact and proxy of the undersigned with respect to such Shares and any Distributions with full power of substitution (such power of attorney being deemed to be an irrevocable power coupled with an interest) to the fullest extent of such stockholder’s rights with respect to such Shares and any Distributions (i) to deliver certificates representing Shares (the “Share Certificates”) and any Distributions, or transfer of ownership of such Shares and any Distributions on the account books maintained by The Depository Trust Company (“DTC”), together, in either such case, with all accompanying evidence of transfer and authenticity, to or upon the order of Purchaser, (ii) to present such Shares and any Distributions for transfer on the books of Data Domain, and (iii) to receive all benefits and otherwise exercise all rights of beneficial ownership of such Shares and any Distributions, all in accordance with the terms and subject to the conditions of the Offer.

The undersigned hereby irrevocably appoints each of the designees of Purchaser the attorneys-in-fact and proxies of the undersigned, each with full power of substitution, to the full extent of such stockholder’s rights with respect to the Shares tendered hereby which have been accepted for payment and with respect to any Distributions. The designees of Purchaser will, with respect to the Shares and any associated Distributions for which the appointment is effective, be empowered to exercise all voting and any other rights of such stockholder, as they, in their sole discretion, may deem proper at any annual, special, adjourned or postponed meeting of Data Domain’s stockholders, by written consent in lieu of any such meeting or otherwise. This proxy and power of attorney shall be irrevocable and coupled with an interest in the tendered Shares. Such appointment is effective when, and only to the extent that, Purchaser accepts the Shares tendered with this Letter of Transmittal for payment pursuant to the Offer. Upon the effectiveness of such appointment, without further action, all prior powers of attorney, proxies and consents given by the undersigned with respect to such Shares and any associated Distributions will be revoked and no subsequent powers of attorney, proxies, consents or revocations may be given (and, if given, will not be deemed effective). Purchaser reserves the right to require that, in order for Shares to be deemed validly tendered, immediately upon Purchaser’s acceptance for payment of such Shares, Purchaser must be able to exercise full voting, consent and other rights, to the extent permitted under applicable law, with respect to such Shares and any associated Distributions, including voting at any meeting of stockholders or executing a written consent concerning any matter.

The undersigned hereby represents and warrants that the undersigned has full power and authority to tender, sell, assign and transfer the Shares and any Distributions tendered hereby and, when the same are accepted for payment by Purchaser, Purchaser will acquire good, marketable and unencumbered title thereto, free and clear of all liens, restrictions, charges and encumbrances and the same will not be subject to any adverse claim. The undersigned hereby represents and warrants that the undersigned is the registered owner of the Shares or the Share Certificate(s) have been endorsed to the undersigned in blank or the undersigned is a participant in DTC whose name appears on a security position listing participant as the owner of the Shares. The undersigned will,

upon request, execute and deliver any additional documents deemed by the Depositary or Purchaser to be necessary or desirable to complete the sale, assignment and transfer of the Shares and any Distributions tendered hereby. In addition, the undersigned shall promptly remit and transfer to the Depositary for the account of Purchaser any and all Distributions in respect of the Shares tendered hereby, accompanied by appropriate documentation of transfer and, pending such remittance or appropriate assurance thereof, Purchaser shall be entitled to all rights and privileges as owner of any such Distributions and may withhold the entire purchase price or deduct from the purchase price the amount or value thereof, as determined by Purchaser in its sole discretion.

It is understood that the undersigned will not receive payment for the Shares unless and until the Shares are accepted for payment and until the Share Certificate(s) owned by the undersigned are received by the Depositary at the address set forth above, together with such additional documents as the Depositary may require, or, in the case of Shares held in book-entry form, ownership of Shares is validly transferred on the account books maintained by DTC, and until the same are processed for payment by the Depositary. It is understood that the method of delivery of the Shares, the Share Certificate(s) and all other required documents (including delivery through DTC) is at the option and risk of the undersigned and that the risk of loss of such Shares, Share Certificate(s) and other documents shall pass only after the Depositary has actually received the Shares or Share Certificate(s) (including, in the case of a book-entry transfer, by Book-Entry Confirmation (as defined below)).

All authority conferred or agreed to be conferred pursuant to this Letter of Transmittal shall not be affected by, and shall survive, the death or incapacity of the undersigned and any obligation of the undersigned hereunder shall be binding upon the heirs, executors, administrators, trustees in bankruptcy, personal representatives, successors and assigns of the undersigned. Except as stated in the Offer to Purchase, this tender is irrevocable.

The undersigned understands that the acceptance for payment by Purchaser of Shares tendered pursuant to one of the procedures described in Section 3 of the Offer to Purchase will constitute a binding agreement between the undersigned and Purchaser upon the terms and subject to the conditions of the Offer.

Unless otherwise indicated herein under “Special Payment Instructions”, please issue the check for the purchase price in the name(s) of, and/or return any Share Certificates representing Shares not tendered or accepted for payment to, the registered owner(s) appearing under “Description of Shares Tendered”. Similarly, unless otherwise indicated under “Special Delivery Instructions”, please mail the check for the purchase price and/or return any Share Certificates representing Shares not tendered or accepted for payment (and accompanying documents, as appropriate) to the address(es) of the registered owner(s) appearing under “Description of Shares Tendered”. In the event that both the Special Delivery Instructions and the Special Payment Instructions are completed, please issue the check for the purchase price and/or issue any Share Certificates representing Shares not tendered or accepted for payment (and any accompanying documents, as appropriate) in the name of, and deliver such check and/or return such Share Certificates (and any accompanying documents, as appropriate) to, the person or persons so indicated. Unless otherwise indicated herein in the box titled “Special Payment Instructions”, please credit any Shares tendered hereby or by an Agent’s Message and delivered by book-entry transfer, but which are not purchased, by crediting the account at DTC designated above. The undersigned recognizes that Purchaser has no obligation pursuant to the Special Payment Instructions to transfer any Shares from the name of the registered owner thereof if Purchaser does not accept for payment any of the Shares so tendered.

To be completed ONLY if Share Certificate(s) not tendered or not accepted for payment and/or the check for the purchase price of Shares accepted for payment are to be issued in the name of someone other than the undersigned or if Shares tendered by book-entry transfer which are not accepted for payment are to be returned by credit to an account maintained at DTC other than that designated above.

(Taxpayer Identification or Social Security Number) (See IRS Form W-9 included in this Letter of Transmittal)

To be completed ONLY if Share Certificate(s) not tendered or not accepted for payment and/or the check for the purchase price of Shares accepted for payment are to be sent to someone other than the undersigned or to the undersigned at an address other than that shown in the box titled “Description of Shares Tendered” above.

IMPORTANT — SIGN HERE

(and also complete IRS Form W-9)

(Signature(s) of Stockholder(s))

Dated: , 2009

(Must be signed by registered owner(s) exactly as name(s) appear(s) on Share Certificate(s) or on a security position listing or by person(s) authorized to become registered owner(s) by certificates and documents transmitted herewith. If signature is by trustees, executors, administrators, guardians, attorneys-in-fact, officers of corporations or others acting in a fiduciary or representative capacity, please set forth full title and see Instruction 5. For information concerning signature guarantees, see Instruction 1.)

Name(s):

(Please Print)

Capacity (full title):

Address:

(Include Zip Code)

Daytime Area Code and Telephone Number:

Taxpayer Identification or

Social Security No.:

GUARANTEE OF SIGNATURE(S)

(For use by Eligible Institutions only;

see Instructions 1 and 5)

Name of Firm:

(Include Zip Code)

Authorized Signature:

Name:

(Please Type or Print)

Daytime Area Code and Telephone Number:

Dated: , 2009

1. Guarantee of Signatures. Except as otherwise provided below, all signatures on this Letter of Transmittal must be guaranteed by a financial institution (including most commercial banks, savings and loan associations and brokerage houses) that is a member in good standing of a recognized Medallion Program approved by the Securities Transfer Association, Inc., including the Security Transfer Agents Medallion Program, the New York Stock Exchange Medallion Signature Program and the Stock Exchanges Medallion Program (each, an “Eligible Institution”). Signatures on this Letter of Transmittal need not be guaranteed (a) if this Letter of Transmittal is signed by the registered owner(s) (which term, for purposes of this document, includes any participant in DTC whose name appears on a security position listing as the owner of the Shares) of Shares tendered herewith and such registered owner has not completed the box titled “Special Payment Instructions” or the box titled “Special Delivery Instructions” on this Letter of Transmittal or (b) if such Shares are tendered for the account of an Eligible Institution. See Instruction 5.

2. Delivery of Letter of Transmittal and Certificates or Book-Entry Confirmations. This Letter of Transmittal is to be completed by stockholders either if Share Certificates are to be forwarded herewith or, unless an Agent’s Message is utilized, if tenders are to be made pursuant to the procedures for tender by book-entry transfer set forth in Section 3 of the Offer to Purchase. A manually executed facsimile of this document may be used in lieu of the original. Share Certificates representing all physically tendered Shares, or confirmation of any book-entry transfer into the Depositary’s account at DTC of Shares tendered by book-entry transfer (“Book Entry Confirmation”), as well as this Letter of Transmittal properly completed and duly executed with any required signature guarantees, unless an Agent’s Message in the case of a book-entry transfer is utilized, and any other documents required by this Letter of Transmittal, must be received by the Depositary at one of its addresses set forth herein on or prior to the Expiration Date (as defined in Section 1 of the Offer to Purchase). Please do not send your Share Certificates directly to Purchaser, EMC or Data Domain.

Stockholders whose Share Certificates are not immediately available or who cannot deliver all other required documents to the Depositary on or prior to the Expiration Date or who cannot comply with the procedures for book-entry transfer on a timely basis, may nevertheless tender their Shares by properly completing and duly executing a Notice of Guaranteed Delivery pursuant to the guaranteed delivery procedure set forth in Section 3 of the Offer to Purchase. Pursuant to such procedure: (i) such tender must be made by or through an Eligible Institution, (ii) a properly completed and duly executed Notice of Guaranteed Delivery substantially in the form provided by Purchaser must be received by the Depositary prior to the Expiration Date, and (iii) Share Certificates representing all tendered Shares, in proper form for transfer (or a Book Entry Confirmation with respect to such Shares), as well as a Letter of Transmittal (or facsimile thereof), properly completed and duly executed with any required signature guarantees (unless, in the case of a book-entry transfer, an Agent’s Message is utilized), and all other documents required by this Letter of Transmittal, must be received by the Depositary within three The NASDAQ Global Select Market trading days after the date of execution of such Notice of Guaranteed Delivery.

A properly completed and duly executed Letter of Transmittal (or facsimile thereof) must accompany each such delivery of Share Certificates to the Depositary.

The term “Agent’s Message” means a message, transmitted by DTC to, and received by, the Depositary and forming part of a Book-Entry Confirmation, which states that DTC has received an express acknowledgment from the participant in DTC tendering the Shares which are the subject of such Book-Entry Confirmation that such participant has received and agrees to be bound by the terms of the Letter of Transmittal and that Purchaser may enforce such agreement against the participant.

THE METHOD OF DELIVERY OF THE SHARES, THIS LETTER OF TRANSMITTAL AND ALL OTHER REQUIRED DOCUMENTS, INCLUDING DELIVERY THROUGH DTC, IS AT THE ELECTION AND RISK OF THE TENDERING STOCKHOLDER. DELIVERY OF ALL SUCH DOCUMENTS WILL BE DEEMED MADE AND RISK OF LOSS OF THE SHARE CERTIFICATES

SHALL PASS ONLY WHEN ACTUALLY RECEIVED BY THE DEPOSITARY (INCLUDING, IN THE CASE OF A BOOK-ENTRY TRANSFER, BY BOOK-ENTRY CONFIRMATION). IF SUCH DELIVERY IS BY MAIL, IT IS RECOMMENDED THAT ALL SUCH DOCUMENTS BE SENT BY PROPERLY INSURED REGISTERED MAIL WITH RETURN RECEIPT REQUESTED. IN ALL CASES, SUFFICIENT TIME SHOULD BE ALLOWED TO ENSURE TIMELY DELIVERY.

No alternative, conditional or contingent tenders will be accepted and no fractional Shares will be purchased. All tendering stockholders, by execution of this Letter of Transmittal (or facsimile thereof), waive any right to receive any notice of the acceptance of their Shares for payment.

All questions as to validity, form and eligibility of the surrender of any Share Certificate hereunder will be determined by Purchaser (which may delegate power in whole or in part to the Depositary) and such determination shall be final and binding. Purchaser reserves the right to waive any irregularities or defects in the surrender of any Shares or Share Certificate(s). A surrender will not be deemed to have been made until all irregularities have been cured or waived. Purchaser and the Depositary shall make reasonable efforts to notify any person of any defect in any Letter of Transmittal submitted to the Depositary.

3. Inadequate Space. If the space provided herein is inadequate, the certificate numbers and/or the number of Shares should be listed on a separate schedule attached hereto and separately signed on each page thereof in the same manner as this Letter of Transmittal is signed.

4. Partial Tenders (Applicable to Certificate Stockholders Only). If fewer than all the Shares evidenced by any Share Certificate delivered to the Depositary are to be tendered, fill in the number of Shares which are to be tendered in the column titled “Number of Shares Tendered” in the box titled “Description of Shares Tendered”. In such cases, new certificate(s) for the remainder of the Shares that were evidenced by the old certificate(s) but not tendered will be sent to the registered owner, unless otherwise provided in the appropriate box on this Letter of Transmittal, as soon as practicable after the Expiration Date. All Shares represented by Share Certificates delivered to the Depositary will be deemed to have been tendered unless otherwise indicated.

5. Signatures on Letter of Transmittal; Stock Powers and Endorsements. If this Letter of Transmittal is signed by the registered owner(s) of the Shares tendered hereby, the signature(s) must correspond with the name(s) as written on the face of the Share Certificate(s) without alteration or any other change whatsoever.

If any Shares tendered hereby are owned of record by two or more joint owners, all such owners must sign this Letter of Transmittal.

If any tendered Shares are registered in the names of different holder(s), it will be necessary to complete, sign and submit as many separate Letters of Transmittal (or facsimiles thereof) as there are different registrations of such Shares.

If this Letter of Transmittal or any certificates or stock powers are signed by trustees, executors, administrators, guardians, attorneys-in-fact, officers of corporations or others acting in a fiduciary or representative capacity, such persons should so indicate when signing, and proper evidence satisfactory to Purchaser of their authority so to act must be submitted.

If this Letter of Transmittal is signed by the registered owner(s) of the Shares listed and transmitted hereby, no endorsements of Share Certificates or separate stock powers are required unless payment is to be made to, or Share Certificates representing Shares not tendered or accepted for payment are to be issued in the name of, a person other than the registered owner(s). Signatures on such Share Certificates or stock powers must be guaranteed by an Eligible Institution.

If this Letter of Transmittal is signed by a person other than the registered owner(s) of the Share(s) listed, the Share Certificate(s) must be endorsed or accompanied by the appropriate stock powers, in either case, signed exactly as the name or names of the registered owner(s) or holder(s) appear(s) on the Share Certificate(s). Signatures on such Share Certificates or stock powers must be guaranteed by an Eligible Institution.

6. Transfer Taxes. Except as otherwise provided in this Instruction 6, Purchaser will pay any transfer taxes with respect to the transfer and sale of Shares to it or to its order pursuant to the Offer. If, however, payment of the purchase price is to be made to, or (in the circumstances permitted hereby) if Share Certificates not tendered or accepted for payment are to be registered in the name of, any person other than the registered owner(s), or if tendered Share Certificates are registered in the name of any person other than the person signing this Letter of Transmittal, the amount of any transfer taxes (whether imposed on the registered owner(s) or such person) payable on account of the transfer to such person will be deducted from the purchase price unless satisfactory evidence of the payment of such taxes, or exemption therefrom, is submitted.

7. Special Payment and Delivery Instructions. If a check is to be issued in the name of, and/or Share Certificates representing Shares not tendered or accepted for payment are to be issued or returned to, a person other than the signer(s) of this Letter of Transmittal or if a check and/or such certificates are to be mailed to a person other than the signer(s) of this Letter of Transmittal or to an address other than that shown in the box titled “Description of Shares Tendered” above, the appropriate boxes on this Letter of Transmittal should be completed. Stockholders delivering Shares tendered hereby or by Agent’s Message by book-entry transfer may request that Shares not purchased be credited to an account maintained at DTC as such stockholder may designate in the box titled “Special Payment Instructions” herein. If no such instructions are given, all such Shares not purchased will be returned by crediting the same account at DTC as the account from which such Shares were delivered.

8. Requests for Assistance or Additional Copies. Questions or requests for assistance may be directed to the Information Agent at its address and telephone number set forth below or to your broker, dealer, commercial bank or trust company. Additional copies of the Offer to Purchase, this Letter of Transmittal, the Notice of Guaranteed Delivery and other tender offer materials may be obtained from the Information Agent as set forth below, and will be furnished at Purchaser’s expense.

9. Backup Withholding. Under current U.S. federal income tax law, the Depositary may be required under the backup withholding tax rules to withhold a portion of any payments made to certain stockholders (or other payees) pursuant to the Offer. In general, to avoid such backup withholding tax, each tendering stockholder must timely provide the Depositary with such stockholder’s correct taxpayer identification number (“TIN”) on IRS Form W-9 (attached hereto) or otherwise establish a basis for exemption from backup withholding tax (currently imposed at a rate of 28%). Certain stockholders (including, among others, all corporations and certain non-U.S. persons) are exempt from these backup withholding tax requirements. Exempt stockholders that are U.S. persons should furnish their TIN, check the box labeled “Exempt payee”, and sign, date and send the IRS Form W-9 to the Depositary. Non-U.S. persons, including entities, may qualify as exempt recipients by submitting to the Depositary a properly completed IRS Form W-8BEN (or other applicable form), signed under penalties of perjury, attesting to that stockholder’s non-U.S. status. The applicable IRS Form W-8 can be obtained from the IRS or from the Depositary. If a stockholder is an individual who is a U.S. citizen or resident, the TIN is generally his or her social security number. If the Depositary is not provided with the correct TIN by a stockholder, a $50 penalty may be imposed by the IRS and/or payments made to that stockholder pursuant to the Offer may be subject to backup withholding tax. Failure to comply truthfully with the backup withholding tax requirements may also result in the imposition of criminal and/or civil fines and penalties. See IRS Form W-9 for additional instructions.

If backup withholding tax applies, the Depositary is required to withhold on any payments made to the stockholder (or other payee) pursuant to the Offer. Backup withholding tax is not an additional tax. A stockholder subject to the backup withholding tax rules will be allowed a credit of the amount withheld against such stockholder’s U.S. federal income tax liability and, if backup withholding tax results in an overpayment of U.S. federal income tax, such stockholder may be entitled to a refund, provided that the requisite information is correctly furnished to the IRS in a timely manner. Stockholders should consult their own tax advisors regarding the backup withholding tax rules.

10. Lost, Destroyed, Mutilated or Stolen Share Certificates. If any Share Certificate has been lost, destroyed, mutilated or stolen, the stockholder should promptly notify Data Domain’s stock transfer agent,

Wells Fargo Shareowner Services, at (800) 468-9716 (toll free). The stockholder will then be instructed as to the steps that must be taken in order to replace the Share Certificate. This Letter of Transmittal and related documents cannot be processed until the procedures for replacing lost, mutilated, destroyed or stolen Share Certificates have been followed.

11. Waiver of Conditions. Subject to the terms and conditions of the Offer (as defined in the Offer to Purchase) and the applicable rules and regulations of the Commission, the conditions of the Offer may be waived by Purchaser in whole or in part at any time and from time to time in its sole discretion.

IMPORTANT: THIS LETTER OF TRANSMITTAL (OR A MANUALLY EXECUTED FACSIMILE COPY THEREOF) OR AN AGENT’S MESSAGE, TOGETHER WITH SHARE CERTIFICATE(S) OR BOOK-ENTRY CONFIRMATION OR A PROPERLY COMPLETED AND DULY EXECUTED NOTICE OF GUARANTEED DELIVERY AND ALL OTHER REQUIRED DOCUMENTS MUST BE RECEIVED BY THE DEPOSITARY ON OR PRIOR TO THE EXPIRATION DATE.

Certification instructions. You must cross out item 2 above if you have been notified by the IRS that you are currently subject to backup withholding because you have failed to report all interest and dividends on your tax return. For real estate transactions, item 2 does not apply. For mortgage interest paid, acquisition or abandonment of secured property, cancellation of debt, contributions to an individual retirement arrangement (IRA), and generally, payments other than interest and dividends, you are not required to sign the Certification, but you must provide your correct TIN. See the instructions on page 4.

General Instructions

Section references are to the Internal Revenue Code unless otherwise noted.

Purpose of Form

A person who is required to file an information return with the IRS must obtain your correct taxpayer identification number (TIN) to report, for example, income paid to you, real estate transactions, mortgage interest you paid, acquisition or abandonment of secured property, cancellation of debt, or contributions you made to an IRA.

Use Form W-9 only if you are a U.S. person (including a resident alien), to provide your correct TIN to the person requesting it (the requester) and, when applicable, to:

1. Certify that the TIN you are giving is correct (or you are waiting for a number to be issued),

2. Certify that you are not subject to backup withholding, or

3. Claim exemption from backup withholding if you are a U.S. exempt payee. If applicable, you are also certifying that as a U.S. person, your allocable share of any partnership income from a U.S. trade or business is not subject to the withholding tax on foreign partners’ share of effectively connected income.

Note. If a requester gives you a form other than Form W-9 to request your TIN, you must use the requester’s form if it is substantially similar to this Form W-9.

Definition of a U.S. person. For federal tax purposes, you are considered a U.S. person if you are:

— An individual who is a U.S. citizen or U.S. resident alien,

— A partnership, corporation, company, or association created or organized in the United States or under the laws of the United States,

— An estate (other than a foreign estate), or

— A domestic trust (as defined in Regulations section 301.7701-7).

Special rules for partnerships. Partnerships that conduct a trade or business in the United States are generally required to pay a withholding tax on any foreign partners’ share of income from such business. Further, in certain cases where a Form W-9 has not been received, a partnership is required to presume that a partner is a foreign person, and pay the withholding tax. Therefore, if you are a U.S. person that is a partner in a partnership conducting a trade or business in the United States, provide Form W-9 to the partnership to establish your U.S. status and avoid withholding on your share of partnership income.

The person who gives Form W-9 to the partnership for purposes of establishing its U.S. status and avoiding withholding on its allocable share of net income from the partnership conducting a trade or business in the United States is in the following cases:

— The U.S. owner of a disregarded entity and not the entity,

— The U.S. grantor or other owner of a grantor trust and not the trust, and

— The U.S. trust (other than a grantor trust) and not the beneficiaries of the trust.

Foreign person. If you are a foreign person, do not use Form W-9. Instead, use the appropriate Form W-8 (see Publication 515, Withholding of Tax on Nonresident Aliens and Foreign Entities).

Nonresident alien who becomes a resident alien. Generally, only a nonresident alien individual may use the terms of a tax treaty to reduce or eliminate U.S. tax on certain types of income. However, most tax treaties contain a provision known as a “saving clause.” Exceptions specified in the saving clause may permit an exemption from tax to continue for certain types of income even after the payee has otherwise become a U.S. resident alien for tax purposes.

If you are a U.S. resident alien who is relying on an exception contained in the saving clause of a tax treaty to claim an exemption from U.S. tax on certain types of income, you must attach a statement to Form W-9 that specifies the following five items:

1. The treaty country. Generally, this must be the same treaty under which you claimed exemption from tax as a nonresident alien.

2. The treaty article addressing the income.

3. The article number (or location) in the tax treaty that contains the saving clause and its exceptions.

4. The type and amount of income that qualifies for the exemption from tax.

5. Sufficient facts to justify the exemption from tax under the terms of the treaty article.

Example. Article 20 of the U.S.-China income tax treaty allows an exemption from tax for scholarship income received by a Chinese student temporarily present in the United States. Under U.S. law, this student will become a resident alien for tax purposes if his or her stay in the United States exceeds 5 calendar years. However, paragraph 2 of the first Protocol to the U.S.-China treaty (dated April 30, 1984) allows the provisions of Article 20 to continue to apply even after the Chinese student becomes a resident alien of the United States. A Chinese student who qualifies for this exception (under paragraph 2 of the first protocol) and is relying on this exception to claim an exemption from tax on his or her scholarship or fellowship income would attach to Form W-9 a statement that includes the information described above to support that exemption.

If you are a nonresident alien or a foreign entity not subject to backup withholding, give the requester the appropriate completed Form W-8.

What is backup withholding? Persons making certain payments to you must under certain conditions withhold and pay to the IRS 28% of such payments. This is called “backup withholding.” Payments that may be subject to backup withholding include interest, tax-exempt interest, dividends, broker and barter exchange transactions, rents, royalties, nonemployee pay, and certain payments from fishing boat operators. Real estate transactions are not subject to backup withholding.

You will not be subject to backup withholding on payments you receive if you give the requester your correct TIN, make the proper certifications, and report all your taxable interest and dividends on your tax return.

Payments you receive will be subject to backup withholding if:

1. You do not furnish your TIN to the requester,

2. You do not certify your TIN when required (see the Part II instructions on page 3 for details),

3. The IRS tells the requester that you furnished an incorrect TIN,

4. The IRS tells you that you are subject to backup withholding because you did not report all your interest and dividends on your tax return (for reportable interest and dividends only), or

5. You do not certify to the requester that you are not subject to backup withholding under 4 above (for reportable interest and dividend accounts opened after 1983 only).

Certain payees and payments are exempt from backup withholding. See the instructions below and the separate Instructions for the Requester of Form W-9.

Also see Special rules for partnerships on page 1.

Penalties

Failure to furnish TIN. If you fail to furnish your correct TIN to a requester, you are subject to a penalty of $50 for each such failure unless your failure is due to reasonable cause and not to willful neglect.

Civil penalty for false information with respect to withholding. If you make a false statement with no reasonable basis that results in no backup withholding, you are subject to a $500 penalty.

Criminal penalty for falsifying information. Willfully falsifying certifications or affirmations may subject you to criminal penalties including fines and/or imprisonment.

Misuse of TINs. If the requester discloses or uses TINs in violation of federal law, the requester may be subject to civil and criminal penalties.

Specific Instructions

Name

If you are an individual, you must generally enter the name shown on your income tax return. However, if you have changed your last name, for instance, due to marriage without informing the Social Security Administration of the name change, enter your first name, the last name shown on your social security card, and your new last name.

If the account is in joint names, list first, and then circle, the name of the person or entity whose number you entered in Part I of the form.

Sole proprietor. Enter your individual name as shown on your income tax return on the “Name” line. You may enter your business, trade, or “doing business as (DBA)” name on the “Business name” line.

Limited liability company (LLC). Check the “Limited liability company” box only and enter the appropriate code for the tax classification (“D” for disregarded entity, “C” for corporation, “P” for partnership) in the space provided.

For a single-member LLC (including a foreign LLC with a domestic owner) that is disregarded as an entity separate from its owner under Regulations section 301.7701-3, enter the owner’s name on the “Name” line. Enter the LLC’s name on the “Business name” line.

For an LLC classified as a partnership or a corporation, enter the LLC’s name on the “Name” line and any business, trade, or DBA name on the “Business name” line.

Other entities. Enter your business name as shown on required federal tax documents on the “Name” line. This name should match the name shown on the charter or other legal document creating the entity. You may enter any business, trade, or DBA name on the “Business name” line.

Note. You are requested to check the appropriate box for your status (individual/sole proprietor, corporation, etc.).

Exempt Payee

If you are exempt from backup withholding, enter your name as described above and check the appropriate box for your status, then check the “Exempt payee” box in the line following the business name, sign and date the form.

Generally, individuals (including sole proprietors) are not exempt from backup withholding. Corporations are exempt from backup withholding for certain payments, such as interest and dividends.

Note. If you are exempt from backup withholding, you should still complete this form to avoid possible erroneous backup withholding.

The following payees are exempt from backup withholding:

1. An organization exempt from tax under section 501(a), any IRA, or a custodial account under section 403(b)(7) if the account satisfies the requirements of section 401(f)(2),

2. The United States or any of its agencies or instrumentalities,

3. A state, the District of Columbia, a possession of the United States, or any of their political subdivisions or instrumentalities,

4. A foreign government or any of its political subdivisions, agencies, or instrumentalities, or

5. An international organization or any of its agencies or instrumentalities.

Other payees that may be exempt from backup withholding include:

6. A corporation,

7. A foreign central bank of issue,

8. A dealer in securities or commodities required to register in the United States, the District of Columbia, or a possession of the United States,

9. A futures commission merchant registered with the Commodity Futures Trading Commission,

10. A real estate investment trust,

11. An entity registered at all times during the tax year under the Investment Company Act of 1940,

12. A common trust fund operated by a bank under section 584(a),

13. A financial institution,

14. A middleman known in the investment community as a nominee or custodian, or

15. A trust exempt from tax under section 664 or described in section 4947.

The chart below shows types of payments that may be exempt from backup withholding. The chart applies to the exempt payees listed above, 1 through 15.


IF the payment is for . . .		THEN the payment is exempt for . . .
Interest and dividend payments		All exempt payees except for 9
Broker transactions		Exempt payees 1 through 13. Also, a person registered under the Investment Advisers Act of 1940 who regularly acts as a broker
Barter exchange transactions and patronage dividends		Exempt payees 1 through 5
Payments over $600 required to be reported and direct sales over $5,000 ¹		Generally, exempt payees 1 through 7²

¹	See Form 1099-MISC, Miscellaneous Income, and its instructions.

However, the following payments made to a corporation (including gross proceeds paid to an attorney under section 6045(f), even if the attorney is a corporation) and reportable on Form 1099-MISC are not exempt from backup withholding: medical and health care payments, attorneys’ fees, and payments for services paid by a federal executive agency.

Part I. Taxpayer Identification Number (TIN)

Enter your TIN in the appropriate box. If you are a resident alien and you do not have and are not eligible to get an SSN, your TIN is your IRS individual taxpayer identification number (ITIN). Enter it in the social security number box. If you do not have an ITIN, see How to get a TIN below.

If you are a sole proprietor and you have an EIN, you may enter either your SSN or EIN. However, the IRS prefers that you use your SSN.

If you are a single-member LLC that is disregarded as an entity separate from its owner (see Limited liability company (LLC) on page 2), enter the owner’s SSN (or EIN, if the owner has one). Do not enter the disregarded entity’s EIN. If the LLC is classified as a corporation or partnership, enter the entity’s EIN.

Note. See the chart on page 4 for further clarification of name and TIN combinations.

How to get a TIN. If you do not have a TIN, apply for one immediately. To apply for an SSN, get Form SS-5, Application for a Social Security Card, from your local Social Security Administration office or get this form online at www.ssa.gov. You may also get this form by calling 1-800-772-1213. Use Form W-7, Application for IRS Individual Taxpayer Identification Number, to apply for an ITIN, or Form SS-4, Application for Employer Identification Number, to apply for an EIN. You can apply for an EIN online by accessing the IRS website at www.irs.gov/businesses and clicking on Employer Identification Number (EIN) under Starting a Business. You can get Forms W-7 and SS-4 from the IRS by visiting www.irs.gov or by calling 1-800-TAX-FORM (1-800-829-3676).

If you are asked to complete Form W-9 but do not have a TIN, write “Applied For” in the space for the TIN, sign and date the form, and give it to the requester. For interest and dividend payments, and certain payments made with respect to readily tradable instruments, generally you will have 60 days to get a TIN and give it to the requester before you are subject to backup withholding on payments. The 60-day rule does not apply to other types of payments. You will be subject to backup withholding on all such payments until you provide your TIN to the requester.

Note. Entering “Applied For” means that you have already applied for a TIN or that you intend to apply for one soon.

Caution: A disregarded domestic entity that has a foreign owner must use the appropriate Form W-8.

Part II. Certification

To establish to the withholding agent that you are a U.S. person, or resident alien, sign Form W-9. You may be requested to sign by the withholding agent even if items 1, 4, and 5 below indicate otherwise.

For a joint account, only the person whose TIN is shown in Part I should sign (when required). Exempt payees, see Exempt Payee on page 2.

Signature requirements. Complete the certification as indicated in 1 through 5 below.

1. Interest, dividend, and barter exchange accounts opened before 1984 and broker accounts considered active during 1983. You must give your correct TIN, but you do not have to sign the certification.

2. Interest, dividend, broker, and barter exchange accounts opened after 1983 and broker accounts considered inactive during 1983. You must sign the certification or backup withholding will apply. If you are subject to backup withholding and you are merely providing your correct TIN to the requester, you must cross out item 2 in the certification before signing the form.

3. Real estate transactions. You must sign the certification. You may cross out item 2 of the certification.

4. Other payments. You must give your correct TIN, but you do not have to sign the certification unless you have been notified that you have previously given an incorrect TIN. “Other payments” include payments made in the course of the requester’s trade or business for rents, royalties, goods (other than bills for merchandise), medical and health care services (including payments to corporations), payments to a nonemployee for services, payments to certain fishing boat crew members and fishermen, and gross proceeds paid to attorneys (including payments to corporations).

5. Mortgage interest paid by you, acquisition or abandonment of secured property, cancellation of debt, qualified tuition program payments (under section 529), IRA, Coverdell ESA, Archer MSA or HSA contributions or distributions, and pension distributions. You must give your correct TIN, but you do not have to sign the certification.


What Name and Number To Give the Requester
	For this type of account:	Give name and SSN of:
1.	Individual	The individual
2.	Two or more individuals (joint account)	The actual owner of the account or, if combined funds, the first individual on the account ¹
3.	Custodian account of a minor (Uniform Gift to Minors Act)	The minor ²
4.	a. The usual revocable savings trust (grantor is also trustee)	The grantor-trustee ¹
	b. So-called trust account that is not a legal or valid trust under state law	The actual owner ¹
5.	Sole proprietorship or disregarded entity owned by an individual	The owner ³
	For this type of account:	Give name and EIN of:
6.	Disregarded entity not owned by an individual	The owner
7.	A valid trust, estate, or pension trust	Legal entity ⁴
8.	Corporate or LLC electing corporate status on Form 8832	The corporation
9.	Association, club, religious, charitable, educational, or other tax-exempt organization	The organization
10.	Partnership or multi-member LLC	The partnership
11.	A broker or registered nominee	The broker or nominee
12.	Account with the Department of Agriculture in the name of a public entity (such as a state or local government, school district, or prison) that receives agricultural program payments	The public entity

¹	List first and circle the name of the person whose number you furnish. If only one person on a joint account has an SSN, that person’s number must be furnished.

²	Circle the minor’s name and furnish the minor’s SSN.

³	You must show your individual name and you may also enter your business or “DBA” name on the second name line. You may use either your SSN or EIN (if you have one), but the IRS encourages you to use your SSN.

⁴	List first and circle the name of the trust, estate, or pension trust. (Do not furnish the TIN of the personal representative or trustee unless the legal entity itself is not designated in the account title.) Also see Special rules for partnerships on page 1.

Note. If no name is circled when more than one name is listed, the number will be considered to be that of the first name listed.

Secure Your Tax Records from Identity Theft

Identity theft occurs when someone uses your personal information such as your name, social security number (SSN), or other identifying information, without your permission, to commit fraud or other crimes. An identity thief may use your SSN to get a job or may file a tax return using your SSN to receive a refund.

To reduce your risk:

— Protect your SSN,

— Ensure your employer is protecting your SSN, and

— Be careful when choosing a tax preparer.

Call the IRS at 1-800-829-1040 if you think your identity has been used inappropriately for tax purposes.

Victims of identity theft who are experiencing economic harm or a system problem, or are seeking help in resolving tax problems that have not been resolved through normal channels, may be eligible for Taxpayer Advocate Service (TAS) assistance. You can reach TAS by calling the TAS toll-free case intake line at 1-877-777-4778 or TTY/TDD 1-800-829-4059.

Protect yourself from suspicious emails or phishing schemes. Phishing is the creation and use of email and websites designed to mimic legitimate business emails and websites. The most common act is sending an email to a user falsely claiming to be an established legitimate enterprise in an attempt to scam the user into surrendering private information that will be used for identity theft.

The IRS does not initiate contacts with taxpayers via emails. Also, the IRS does not request personal detailed information through email or ask taxpayers for the PIN numbers, passwords, or similar secret access information for their credit card, bank, or other financial accounts.

If you receive an unsolicited email claiming to be from the IRS, forward this message to phishing@irs.gov. You may also report misuse of the IRS name, logo, or other IRS personal property to the Treasury Inspector General for Tax Administration at 1-800-366-4484. You can forward suspicious emails to the Federal Trade Commission at: spam@uce.gov or contact them at www.consumer.gov/idtheft or 1-877-IDTHEFT(438-4338).

Visit the IRS website at www.irs.gov to learn more about identity theft and how to reduce your risk.

Section 6109 of the Internal Revenue Code requires you to provide your correct TIN to persons who must file information returns with the IRS to report interest, dividends, and certain other income paid to you, mortgage interest you paid, the acquisition or abandonment of secured property, cancellation of debt, or contributions you made to an IRA, or Archer MSA or HSA. The IRS uses the numbers for identification purposes and to help verify the accuracy of your tax return. The IRS may also provide this information to the Department of Justice for civil and criminal litigation, and to cities, states, the District of Columbia, and U.S. possessions to carry out their tax laws. We may also disclose this information to other countries under a tax treaty, to federal and state agencies to enforce federal nontax criminal laws, or to federal law enforcement and intelligence agencies to combat terrorism.

You must provide your TIN whether or not you are required to file a tax return. Payers must generally withhold 28% of taxable interest, dividend, and certain other payments to a payee who does not give a TIN to a payer. Certain penalties may also apply.

Any questions or requests for assistance or additional copies of the Offer to Purchase and the Letter of Transmittal may be directed to the Information Agent at its telephone number and location listed below. You may also contact your broker, dealer, commercial bank or trust company or other nominee for assistance concerning the Offer.


*By Mail:*		*By Overnight Courier or by Hand:*

BNY Mellon Shareowner Services Corporate Action Division P.O. Box 3301 South Hackensack, NJ 07606		BNY Mellon Shareowner Services Attn: Corporate Action Dept., 27th Floor 480 Washington Boulevard Jersey City, NJ 07310


DESCRIPTION OF SHARES TENDERED
Name(s) and Address(es) of Registered Owner(s) (If blank, please fill in exactly as name(s) appear(s) on share certificate(s))	Shares Tendered (Attach additional list if necessary)
	Shares Certificate Number(s)*	Total Number of Shares Represented By Shares Certificate(s)*	Number of Shares Tendered**







	Total Shares
* Need not be completed by book-entry stockholders. ** Unless otherwise indicated, it will be assumed that all shares of common stock, par value $0.0001 per share, of Data Domain, Inc. represented by certificates described above are being tendered hereby. See Instruction 4.


*By Mail:*		*By Overnight Courier or by Hand:*

BNY Mellon Shareowner Services Corporate Action Division P.O. Box 3301 South Hackensack, NJ 07606		BNY Mellon Shareowner Services Attn: Corporate Action Dept., 27th Floor 480 Washington Boulevard Jersey City, NJ 07310


Part II		Certification


Sign Here		Signature of U.S. person u		Date u