GUARANTY

Exhibit 10.6

GUARANTY

Property Commonly Known as

“Data Center Facility, ACC4, Ashburn, Virginia”

THIS GUARANTY (“ Guaranty ”) made as of February 10, 2009, by TARANTULA VENTURES LLC, a Delaware limited liability company (“ Guarantor ”), to and for the benefit of KEYBANK NATIONAL ASSOCIATION (“ KeyBank ”), a national banking association, as Agent (“ Agent ”), and KeyBank and the other lenders now or hereafter a party to the Credit Agreement (as hereinafter defined) (the “ Lenders ”) (Agent and the Lenders, and their successors and assigns, are hereinafter referred to collectively as the “ Credit Parties ”).

R E C I T A L S

WHEREAS, the Guarantor is the direct owner of certain real property more particularly described in that certain Construction Mortgage, Assignment of Leases and Rents and Fixture Filing dated as of December 20, 2007 from Owner to KeyBank, as agent under a construction loan facility, recorded in the Official Records of Cook County, Illinois as Document No. 0800360019, as amended by that certain First Amendment to Construction Mortgage, Assignment of Leases and Rents and Fixture Filing and Assignment of Leases and Rents dated of even date herewith (as modified and amended from time to time, the “CH1 Mortgage”; such real property is hereinafter referred to as the “Land”; the Land, together with all improvements now or hereafter located in, on or under the Land, collectively, the “Property”);

WHEREAS, Grizzly Ventures LLC, a Delaware limited liability company (“Borrower”), DuPont Fabros Technology, L.P., a Maryland limited partnership (“Parent Guarantor”), KeyBank, as Agent, and the Lenders entered into that certain Credit Agreement dated as of October 24, 2008, as amended by that certain First Amendment to Credit Agreement and Other Loan Documents (the “Term Loan Amendment”) dated as of even date herewith between Borrower, Parent Guarantor, Guarantor, KeyBank, as Agent and the Lenders (as the same may be varied, extended, supplemented, consolidated, replaced, increased, renewed, modified or amended from time to time, the “Credit Agreement”);

WHEREAS, pursuant to the Credit Agreement, Lenders made a loan to Borrower in the original principal amount of $100,000,000.00, increasable up to $250,000,000.00 in accordance with the terms of the Credit Agreement (the “Loan”), which Loan is evidenced by, among other things, the Notes made by Borrower to the order of Lenders and delivered from time to time under the Credit Agreement (together with all amendments, modifications, consolidations, increases, supplements and extensions thereof, and together with the new notes and replacement notes executed in connection with the commitment increase pursuant to the Term Loan Amendment (as hereinafter defined), collectively, the “Notes”);

WHEREAS, Borrower desires to increase the “Total Commitment” under the Credit Agreement;

WHEREAS, Guarantor will derive material financial benefit from the increase in the Total Commitment under the Credit Agreement pursuant to the Term Loan Amendment;

WHEREAS, the Credit Parties have relied on the statements and agreements contained herein in agreeing to execute the Term Loan Amendment; and

WHEREAS, in order to induce additional lenders to join in funding the increase of the Total Commitment, Lenders require that Guarantor enter into this Guaranty as a condition precedent to the execution of the Term Loan Amendment by Lenders.

AGREEMENTS

NOW, THEREFORE, intending to be legally bound, Guarantor, in consideration of the matters described in the foregoing Recitals, which Recitals are incorporated herein and made a part hereof, and for other good and valuable consideration the receipt and sufficiency of which are acknowledged, hereby covenants and agrees for the benefit of the Credit Parties and their respective successors, indorsees, transferees, participants and assigns as follows:

1. Guarantor, absolutely, unconditionally, and irrevocably guarantees:

(a) the full and prompt payment of the principal of and interest on the Notes when due, whether at stated maturity, upon acceleration or otherwise, and at all times thereafter, and the full and prompt payment of all sums which may now be or may hereafter become due and owing under the Notes, the Credit Agreement and the other Loan Documents;

(b) the full, complete and punctual observance, performance and satisfaction of all of the other obligations, duties, covenants and agreements of Borrower under the Credit Agreement and the other Loan Documents;

(c) the full and prompt payment of any Enforcement Costs (as hereinafter defined in Section 7 hereof); and

(d) the full and prompt payment of all of the Hedge Obligations (as defined in the Credit Agreement).

All amounts due, debts, liabilities, payment obligations and other obligations described in subsections (a) through (d) of this Section 1 are referred to herein as the “Guaranteed Obligations.”

2. In the event of any default by Borrower in the payment or performance of the Guaranteed Obligations and the expiration of any applicable cure or grace period, Guarantor agrees, on demand by Agent or the Credit Parties (which demand may be made concurrently with notice to Borrower that Borrower is in default of its obligations), to pay and perform all the Guaranteed Obligations regardless of any defense, right of setoff or claims which Borrower, Guarantor or Parent Guarantor may have against any of the Credit Parties. The Credit Parties shall have the right, at their option, either before, during or after commencing foreclosure or sale proceedings, as the case may be, and before, during or after pursuing any other right or remedy against Borrower, Guarantor or Parent Guarantor, to perform any and all of the Guaranteed Obligations by or through any agent of its selection, all as the Credit Parties in their sole discretion deem proper, and Guarantor shall indemnify and hold the Credit Parties free and harmless of, and against any and all loss, damage, cost, expense, injury, or liability the Credit Parties may suffer or incur in connection with the exercise of their rights under this Guaranty or

2

the performance of the Guaranteed Obligations. Furthermore, the Credit Parties shall not have any obligation to protect or insure any collateral for the Loan, nor shall the Credit Parties have any obligation to perfect their security interest in any collateral for the Loan.

All of the remedies set forth herein and/or provided for in any of the Loan Documents or at law or equity shall be available to the Credit Parties, and the choice by the Credit Parties of one such alternative over another shall not be subject to question or challenge by Guarantor or any other Person, nor shall any such choice be asserted as a defense, setoff, or failure to mitigate damages in any action, proceeding, or counteraction by the Credit Parties to recover or seeking any other remedy under this Guaranty, nor shall such choice preclude the Credit Parties from subsequently electing to exercise a different remedy. The parties have agreed to the alternative remedies hereinabove specified in part because they recognize that the choice of remedies in the event of a failure hereunder will necessarily be and should properly be a matter of good faith business judgment, which the passage of time and events may or may not prove to have been the best choice to maximize recovery by the Credit Parties at the lowest cost to Borrower, Parent Guarantor and/or Guarantor. It is the intention of the parties that such good faith choice by the Credit Parties be given conclusive effect regardless of such subsequent developments.

3. Guarantor hereby agrees that its obligations hereunder shall not be affected or impaired by, and hereby waives and agrees not to assert or take advantage of any defense based on:

(a) (i) any change in the amount, interest rate or due date or other term of any of the obligations hereby guaranteed, (ii) any change in the time, place or manner of payment of all or any portion of the obligations hereby guaranteed, (iii) any amendment or waiver of, or consent to the departure from or other indulgence with respect to, the Credit Agreement, any other Loan Document, or any other document or instrument evidencing or relating to any obligations hereby guaranteed, or (iv) any waiver, renewal, extension, addition, or supplement to, or deletion from, or any other action or inaction under or in respect of, the Credit Agreement, any of the other Loan Documents, or any other documents, instruments or agreements relating to the obligations hereby guaranteed or any other instrument or agreement referred to therein or evidencing any obligations hereby guaranteed or any assignment or transfer of any of the foregoing;

(b) any subordination of the payment of the obligations hereby guaranteed to the payment of any other liability of Borrower or any other Person;

(c) any act or failure to act by Borrower or any other Person which may adversely affect Guarantor’s subrogation rights, if any, against Borrower or any other Person to recover payments made under this Guaranty;

(d) any nonperfection or impairment of any security interest or other lien on any collateral, if any, securing in any way any of the obligations hereby guaranteed or any failure on the part of the Credit Parties to ascertain the extent or nature of any collateral or any insurance or other rights with respect thereto, or the liability of any party liable under the Loan Documents or the obligations evidenced or secured thereby;

3

(e) any application of sums paid by Borrower or any other Person with respect to the Guaranteed Obligations, regardless of what liabilities of Borrower remain unpaid;

(f) any defense of Borrower, including without limitation, the invalidity, illegality or unenforceability of any of the Guaranteed Obligations;

(g) either with or without notice to Guarantor, any renewal, extension, modification, amendment or another changes in the Guaranteed Obligations, including but not limited to any material alteration of the terms of payment or performance of the Guaranteed Obligations;

(h) any statute of limitations in any action hereunder or for the collection of the Notes or for the payment or performance of any obligation hereby guaranteed;

(i) the incapacity, lack of authority, death or disability of Borrower or any other Person or entity, or the failure of the Credit Parties to file or enforce a claim against the estate (either in administration, bankruptcy or in any other proceeding) of Borrower or Guarantor or any other Person;

(j) the dissolution or termination of existence of Borrower, Guarantor or any other Person;

(k) the voluntary or involuntary liquidation, sale or other disposition of all or substantially all of the assets of Borrower or Guarantor or any other Person;

(l) the voluntary or involuntary receivership, insolvency, bankruptcy, assignment for the benefit of creditors, reorganization, assignment, composition, or readjustment of, or any similar proceeding affecting, Borrower or Guarantor or any other Person, or any of Borrower’s or Guarantor’s or any other Person’s properties or assets;

(m) an assertion or claim that the automatic stay provided by 11 U.S.C. §362 (arising upon the voluntary or involuntary bankruptcy proceeding of Borrower) or any other stay provided under any other debtor relief law (whether statutory, common law, case law or otherwise) of any jurisdiction whatsoever, now or hereafter in effect, which may be or become applicable, shall operate or be interpreted to stay, interdict, condition, reduce or inhibit the ability of the Credit Parties to enforce any of their rights, whether now or hereafter required, which the Credit Parties may have against Guarantor or Parent Guarantor or any collateral for the Loan;

(n) any right or claim of right to cause a marshaling of the assets of Borrower, Parent Guarantor or Guarantor;

(o) the damage, destruction, condemnation, foreclosure or surrender of all or any part of any collateral or the Property or any of the improvements located thereon;

(p) the failure of the Credit Parties to give notice of the existence, creation or incurring of any new or additional indebtedness or obligation of Borrower or of any action or nonaction on the part of any other person whomsoever in connection with any obligation hereby guaranteed;

4

(q) any failure or delay of the Credit Parties to commence an action against Borrower or any other Person, to assert or enforce any remedies against Borrower or any other Person under the Notes or the other Loan Documents, or to realize upon any security;

(r) any failure of any duty on the part of the Credit Parties to disclose to Guarantor any facts they may now or hereafter know regarding Borrower (including, without limitation Borrower’s financial condition), any other person or entity, any collateral, or any other assets or liabilities of such person or entity, whether such facts materially increase the risk to Guarantor or not (it being agreed that Guarantor assumes responsibility for being informed with respect to such information);

(s) failure to accept or give notice of acceptance of this Guaranty by the Credit Parties;

(t) failure to make or give notice of presentment and demand for payment of any of the indebtedness or performance of any of the obligations hereby guaranteed;

(u) failure to make or give protest and notice of dishonor or of default to Guarantor or to any other party with respect to the indebtedness or performance of obligations hereby guaranteed;

(v) any and all other notices whatsoever to which Guarantor might otherwise be entitled;

(w) any lack of diligence by the Credit Parties in collection, protection or realization upon any collateral securing the payment of the indebtedness or performance of obligations hereby guaranteed;

(x) the invalidity or unenforceability of the Notes, or any of the other Loan Documents, or any assignment or transfer of the foregoing;

(y) the compromise, settlement, release or termination of any or all of the obligations of Borrower or Parent Guarantor under the Notes or the other Loan Documents;

(z) any transfer by Borrower or any other Person of all or any part of the security encumbered by the Loan Documents;

(aa) any right to require the Credit Parties to proceed against Borrower or any other Person or to proceed against or exhaust any security held by the Credit Parties at any time or to pursue any other remedy in the Credit Parties’ power or under any other agreement before proceeding against Guarantor hereunder or under any other Loan Document;

(bb) the failure of the Credit Parties to perfect any security or to extend or renew the perfection of any security;

(cc) any principle or provision of law, statutory or otherwise, which is or might be in conflict with the terms and provisions of this Guaranty;

5

(dd) any inaccuracy of any representation or other provision contained in any Loan Document;

(ee) any sale or assignment of the Loan Documents, or any interest therein;

(ff) any and all rights, benefits and defenses which might otherwise be available under the provisions of any other applicable statues, rules or common law principals or provisions which might operate to limit Guarantor’s liability under, or the enforcement of, this Guaranty; or

(gg) to the fullest extent permitted by law, any other legal, equitable or surety defenses whatsoever to which Guarantor might otherwise be entitled, it being the intention that the obligations of Guarantor hereunder are absolute, unconditional and irrevocable.

Guarantor understands that the exercise by the Credit Parties of certain rights and remedies may affect or eliminate Guarantor’s right of subrogation against Borrower or Parent Guarantor and that Guarantor may therefore incur partially or totally nonreimbursable liability hereunder. Nevertheless, Guarantor hereby authorizes and empowers the Credit Parties, their successors, endorsees and assigns, to exercise in its or their sole discretion, any rights and remedies, or any combination thereof, which may then be available, including, without limitation, any remedies against Borrower or Parent Guarantor with respect to the Notes, it being the purpose and intent of Guarantor that the obligations hereunder shall be absolute, continuing, independent and unconditional under any and all circumstances.

4. Guarantor hereby consents and agrees that the Credit Parties may at any time, and from time to time, without thereby releasing Guarantor from any liability hereunder and without notice to or further consent from Guarantor or any other Person, either with or without consideration: release or surrender any lien or other security of any kind or nature whatsoever held by them or by any person, firm or corporation on their behalf or for their account, securing any indebtedness or liability hereby guaranteed; substitute for any collateral so held by them, other collateral of like kind, or of any kind; modify the terms of the Notes or the Loan Documents; extend or renew the Notes for any period; grant releases, compromises and indulgences with respect to the Notes or the Loan Documents and to any persons or entities now or hereafter liable thereunder or hereunder; release any other guarantor, surety, endorser or accommodation party of the Notes or any other Loan Document; or take or fail to take any action of any type whatsoever. No such action which the Credit Parties shall take or fail to take in connection with the Notes or the Loan Documents, or any of them, or any security for the payment of the indebtedness of Borrower to the Credit Parties or for the performance of any obligations or undertakings of Borrower, Parent Guarantor or Guarantor, nor any course of dealing with Borrower or any other Person, shall release Guarantor’s obligations hereunder, affect this Guaranty in any way or afford Guarantor any recourse against the Credit Parties. The provisions of this Guaranty shall extend and be applicable to all replacements, supplements, renewals, amendments, extensions, consolidations, restatements and modifications of the Notes and the other Loan Documents, and any and all references herein to the Notes and the other Loan Documents shall be deemed to include any such replacements, supplements, renewals, extensions, amendments, consolidations, restatements or modifications thereof. Without limiting the generality of the foregoing, Guarantor acknowledges the terms of Section 18 of the Credit Agreement and agrees that this Guaranty shall extend and be applicable to each new or

6

replacement note delivered by Borrower pursuant thereto without notice to or further consent from Guarantor. Guarantor acknowledges that no representations of any kind whatsoever have been made by the Credit Parties. No modification or waiver of any of the provisions of this Guaranty shall be binding upon the Credit Parties except as expressly set forth in a writing duly signed and delivered by Agent in accordance with the provisions of the Credit Agreement.

5. This is an absolute, present and continuing guaranty of payment and performance and not of collection. Guarantor agrees that this Guaranty may be enforced by the Credit Parties without the necessity at any time of resorting to or exhausting any other security or collateral given in connection herewith or with the Notes, Credi