FORM OF VOTING AGREEMENT

Exhibit 99.1

Exhibit A

FORM OF VOTING AGREEMENT

     THIS VOTING AGREEMENT (this “ Agreement ”) is made and entered into as of May 20, 2009 by and between NetApp, Inc., a Delaware corporation ( “ Parent ” ), and the undersigned Stockholder (the “ Stockholder ”) of Data Domain, Inc., a Delaware corporation (the “ Company ”).

WITNESSETH:

     WHEREAS, Parent, Kentucky Merger Sub One Corporation, a Delaware corporation and wholly-owned subsidiary of Parent (“ Merger Sub One ”), Derby Merger Sub Two LLC, a Delaware limited liability company and wholly-owned subsidiary of Parent (“ Merger Sub Two ”), and the Company have entered into an Agreement and Plan of Merger of even date herewith (the “ Merger Agreement ”), which provides for, among other things, the merger of Merger Sub One with and into the Company (the “ First Step Merger ” or the “ Merger ”), pursuant to which all outstanding shares of capital stock of the Company will be converted into the right to receive the consideration set forth in the Merger Agreement. If the opinions described in Section 6.18 of the Merger Agreement are delivered, as soon as practicable following the First Step Merger and as a single integrated transaction, Parent will cause the Company to merge with and into Merger Sub Two with Merger Sub Two continuing as the surviving entity (the “ Second Step Merger ” ; provided that, if the Second Step Merger does occurs, the Second Step Merger shall be included in the meaning of the term, the “ Merger ”).

     WHEREAS, the Stockholder is the beneficial owner (as defined in Rule 13d-3 under the Securities Exchange Act of 1934, as amended (the “ Exchange Act ”)) of that number of shares of the outstanding capital stock of the Company, and the holder of options to purchase such number of shares of capital stock of the Company, in each case, as set forth on the signature page of this Agreement.

     WHEREAS, as a condition and inducement to the willingness of Parent, Merger Sub One and Merger Sub Two to enter into the Merger Agreement, the Stockholder (in the Stockholder’s capacity as such) has agreed to enter into this Agreement.

     NOW, THEREFORE, intending to be legally bound, the parties hereto agree as follows:

     1.  Certain Definitions . All capitalized terms that are used but not defined herein shall have the respective meanings ascribed to them in the Merger Agreement. For all purposes of and under this Agreement, the following terms shall have the following respective meanings:

          (a) “ Expiration Date ” shall mean the earlier to occur of (i) such date and time as the Merger Agreement shall have been validly terminated pursuant to Article VIII thereof, or (ii) such date and time as the Requisite Merger Approval has been obtained or (iii) the written agreement of the parties hereto to terminate this Agreement.

          (b) “ Person ” shall mean any individual, corporation, limited liability company, general or limited partnership, trust, unincorporated association or other entity of any kind or nature, or any governmental authority.

          (c) “ Shares ” shall mean (i) all equity securities of the Company (including all shares of Company Common Stock, Company Preferred Stock and all Company Stock Awards and any other rights to acquire shares of Company capital stock) owned by the Stockholder as of the date hereof, and (ii) all additional equity securities of the Company (including all additional shares of Company Common Stock, Company Preferred Stock and all additional Company Stock Awards, warrants and other rights to acquire shares of Company capital stock) of which the Stockholder acquires beneficial ownership during the period from the date of this Agreement through the Expiration Date (including by way of stock dividend or distribution, split-up, recapitalization, combination, exchange of shares and the like).

          (d) “ Transfer ” A Person shall be deemed to have effected a “ Transfer ” of a Share if such person directly or indirectly (i) sells, pledges, encumbers, assigns, grants an option with respect to, transfers, tenders or disposes of such Share or any interest in such Share, or (ii) enters into an agreement to effect any of the foregoing.

     2.  Transfer of Shares .

          (a) Transfer Restrictions . The Stockholder shall not Transfer (or cause or permit the Transfer of ) any of the Shares, or enter into any agreement relating thereto, except (i) by selling already-owned Shares either to pay the exercise price upon the exercise of a Company Option or to satisfy the Stockholder’s tax withholding obligation upon the exercise of a Company Option, in each case as permitted by any Company Option Plan, or (ii) by selling already-owned Shares pursuant to 10b5-1 trading plans existing as of the date hereof or (iii) transferring Shares to Affiliates, immediate family members, a trust established for the benefit of Stockholder and/or for the benefit of one or more members of the Stockholder’s immediate family or upon the death of the Stockholder or charitable organizations or in connection with, or solely for the purpose of, personal tax-planning, provided that, as a condition to such Transfer, the recipient agrees to be bound by this Agreement and delivers a Proxy (as defined below) in the form attached hereto as Exhibit A . Any Transfer, or purported Transfer, of Shares in breach or violation of this Agreement shall be void and of no force or effect.

          (b) Transfer of Voting Rights . The Stockholder shall not deposit (or cause or permit the deposit of) any Shares in a voting trust or grant any proxy or enter into any voting agreement or similar agreement in contravention of the obligations of the Stockholder under this Agreement with respect to any of the Shares.

     3.  Agreement to Vote Shares .

          (a) At every meeting of the stockholders of the Company, and at every adjournment or postponement thereof, and on every action or approval by written consent of the stockholders of Company, the Stockholder (in the Stockholder’s capacity as such), to the extent not voted by the Person(s) appointed under the Proxy, shall, or shall cause the holder of record on any applicable record date to, vote the Shares:

               (i) in favor of the adoption of the Merger Agreement, and in favor of each of the other actions contemplated by the Merger Agreement and any action required in furtherance thereof;

               (ii) against approval of any proposal made in opposition to, in competition with, or would result in a breach of, the Merger Agreement or the Merger or any other transactions contemplated by the Merger Agreement; and

               (iii) against any of the following actions (other than those actions that relate to the Merger and any other transactions contemplated by the Merger Agreement): (A) any merger, consolidation, business combination, sale of assets, reorganization or recapitalization of or involving the Company or any of its Subsidiaries, (B) any sale, lease or transfer of any significant part of the assets of the Company or any of its Subsidiaries, (C) any reorganization, recapitalization, dissolution, liquidation or winding up of the Company or any of its Subsidiaries, (D) any material change in the capitalization of the Company or any of its Subsidiaries, or the corporate structure of the Company or any of its Subsidiaries, or (E) any other action that is intended, or could reasonably be expected to, impede, interfere with, delay, postpone, discourage or adversely affect the Merger or any other transactions contemplated by the Merger Agreement.

          (b) In the event that a meeting of the Stockholders of the Company is held, the Stockholder shall, or shall cause the holder of record on any applicable record date to, appear at such meeting or otherwise cause the Shares to be counted as present thereat for purposes of establishing a quorum.

          (c) The Stockholder shall not enter into any agreement or understanding with any Person to vote or give instructions in any manner inconsistent with the terms of this Section 3 .

     4.  Agreement Not to Exercise Appraisal Rights . The Stockholder shall not exercise, and hereby irrevocably and unconditionally waives, any statutory rights (including, without limitation, under Section 262 of the DGCL) to demand appraisal of any Shares that may arise in connection with the Merger.

     5.  Directors and Officers . Notwithstanding any provision of this Agreement to the contrary, nothing in this Agreement shall limit or restrict a Stockholder who is a director or officer of the Company from acting in such capacity or voting, in his capacity as a director or officer of the Company, in the Stockholder’s sole discretion on any matter (it being understood that this Agreement shall apply to the Stockholder solely in the Stockholder’s capacity as a Stockholder of the Company). In this regard, the Stockholder shall not be deemed to make any agreement or understanding in this Agreement in Stockholder’s capacity as a director or officer of the Company.

     6.  Irrevocable Proxy . Concurrently with the execution of this Agreement, the Stockholder shall deliver to Parent a proxy in the form attached hereto as Exhibit A (the “ Proxy ”), which shall be irrevocable to the fullest extent permissible by law, with respect to the Shares.

     7.  Representations and Warranties of the Stockholder .

          (a) Power; Binding Agreement . The Stockholder has full power and authority to execute and deliver this Agreement and the Proxy, to perform the Stockholder’s obligations hereunder and to consummate the transactions contemplated hereby. This Agreement has been duly executed and delivered by the Stockholder, and, assuming this Agreement constitutes a valid and

binding obligation of Parent, Merger Sub One and Merger Sub Two, constitutes a valid and binding obligation of the Stockholder, enforceable against the Stockholder in accordance with its terms.

          (b) No Conflicts . None of the execution and delivery by the Stockholder of this Agreement, the performance by the Stockholder of its obligations hereunder or the consummation by the Stockholder of the transactions contemplated hereby will (i) result in a violation or breach of any agreement to which the Stockholder is a party or by which the Stockholder may be bound, including any voting agreement or voting trust, or (ii) violate any order, writ, injunction, decree, judgment, order, statute, rule, or regulation applicable to the Stockholder.

          (c) Ownership of Shares . The Stockholder (i) is the sole beneficial owner of the shares of Company Common Stock set forth on the signature page of this Agreement, all of which are free and clear of any liens, security interests, pledges or options, proxies or any other rights or encumbrances whatsoever (“ Encumbrances ”) (other than pursuant to the terms of restricted stock agreements as in effect on the date hereof and except any Encumbrances arising under securities laws or arising hereunder), (ii) is the sole holder of Company Options that are exercisable for the number of shares of Company Common Stock set forth on the signature page of this Agreement, all of which Company Options and shares of Company Common Stock issuable upon the exercise of such Company Options are, or in the case of Company Common Stock received upon exercise of an option after the date hereof will be, free and clear of any Encumbrances (except any Encumbrances arising under securities laws or arising hereunder), and (iii) except as set forth on