Reader Eric DeVolder writes that he has made available a program to do
 this. I haven't tried it, but it's downloadable from
 <http://www.simtel.net/pub/pd/44403.html>

 Reader Igor Karp reports that MS-DOS version 5.0 and greater provides
 two interrupts to do this.

   --------D-215F07-----------------------------
   INT 21 - DOS 5+ - ENABLE DRIVE
      AX = 5F07h
      DL = drive number (0=A:)
   Return: CF clear if successful
      CF set on error
      AX = error code (0Fh) (see #0885 at AH=59h)
   Notes:   simply  sets the "valid" bit in the  drive's  CDS
   this function is not supported by Novell DOS 7
   See Also: AH=52h,AX=5F08h"DOS"

   --------D-215F08-----------------------------
   INT 21 - DOS 5+ - DISABLE DRIVE
      AX = 5F08h
      DL = drive number (0=A:)
   Return: CF clear if successful
      CF set on error
      AX = error code (0Fh) (see #0885 at AH=59h)
   Notes:   simply clears the "valid" bit in the drive's  CDS
   this function is not supported by Novell DOS 7