<PAGE>




                                                                   EXHIBIT 10.39

                             TWENTY-FIRST AMENDMENT
                                       TO
                    CONVERTIBLE DEBENTURE PURCHASE AGREEMENT


        THIS TWENTY-FIRST AMENDMENT TO CONVERTIBLE DEBENTURE PURCHASE AGREEMENT
(the "Amendment") is entered into as of March 15, 2002, by and between DISC,
Inc., a California corporation (the "Company"), and MK GVD Fund (the
"Purchaser").
                                R E C I T A L S:

        A. WHEREAS on March 29, 1996 the Company and Purchaser entered into a
Convertible Debenture Purchase Agreement pursuant to which the Company agreed to
sell, and Purchaser agreed to purchase, an aggregate of $1,400,000 in principal
amount of Convertible Debentures, each convertible into shares of the Company's
Preferred Stock, which Agreement was amended as of December 31, 1996, April 11,
1997, December 31, 1997, March 27, 1998, June 30, 1998, September 25, 1998,
December 31, 1998, March 30, 1999, June 30, 1999, September 30, 1999, December
31, 1999, March 31, 2000, June 30, 2000, September 30, 2000, December 29, 2000,
March 30, 2001, June 29, 2001, September 28, 2001, December 31, 2001 and March
6, 2002 to increase the aggregate amount of Convertible Debenture to be
purchased thereunder to $16,455,000.

        B. The Company and Purchaser now seek to amend the Agreement to increase
the total amount of Convertible Debentures which Purchaser agrees to purchase
thereunder.

        NOW, THEREFORE, for good and valuable consideration, receipt of which is
hereby acknowledged, and in consideration of the mutual covenants set forth
herein, the parties hereto agree as follows:

        1. DEFINITIONS. Unless otherwise defined herein, capitalized terms used
in the Amendment shall have the same meanings ascribed to them in the
Convertible Debenture Purchase Agreement.

        2. AMENDMENT TO CONVERTIBLE DEBENTURE PURCHASE AGREEMENT. Section 1.1(a)
of the Convertible Debenture Purchase Agreement is hereby amended to provide
that Purchaser agrees to purchase, and the Company agrees to issue and sell, an
aggregate of $16,655,000 in principal amount of Convertible Debentures, each in
the form and having the terms and conditions set forth in the amended Exhibit B
attached to the Twentieth Amendment to Convertible Debenture Purchase Agreement.

        3. ENTIRE AGREEMENT; AMENDMENT. The Convertible Debenture Purchase
Agreement, as amended by this Amendment, constitutes the full and complete
agreement and understanding between the parties hereto regarding the subject
matter of the Convertible Debenture Purchase Agreement and shall supersede all
prior communications, representations, understandings or agreements, if any,
whether oral or written, concerning the subject matter contained in the
Convertible Debenture Purchase Agreement, as so amended, and that no provision
of the Convertible Debenture Purchase Agreement, as so amended, may be modified,
amended, waived or discharged, in whole or in part, except in accordance with
its terms.

        4. FORCE AND EFFECT. Except as modified by this Amendment, the terms and
provisions of the Convertible Debenture Purchase Agreement are hereby ratified
and confirmed and are and shall remain in full force and effect. Should any
inconsistency arise between this Amendment and the Convertible Debenture
Purchase Agreement as to the specific matters which are the subject of this
Amendment, the terms and conditions of this Amendment shall control. This
Amendment shall be construed to be part of the Convertible

<PAGE>


Debenture Purchase Agreement and shall be deemed incorporated into the
Convertible Debenture Purchase Agreement by this reference.

        5. COUNTERPARTS. This Amendment may be executed in one or more
counterparts, each of which shall be an original but all of which together shall
constitute one instrument.

        IN WITNESS WHEREOF, the Company has caused this Amendment to be executed
in duplicate on its behalf by its duly authorized officer and Purchaser has also
executed this Amendment in duplicate, all as of the day and year indicated
above.

                                  DISC, INC.
                                  a California corporation


                                  By: /s/ Henry Madrid
                                      ------------------------------------------
                                      Henry Madrid
                                      Chief Financial Officer


                                   PURCHASER:
                                   MK GVD FUND


                                   By:/s/ Michael Hoffman
                                      ------------------------------------------
                                      Michael Hoffman, General Partner




                                      -2-