Int 21 Fn 5F07  - DOS 5+ - Enable Drive                                    [D]

   AX = 5F07h
   DL = drive number (0=A:)

Return: CF clear if successful
   CF set on error
       AX = error code (0Fh) (see AH=59h)

Note:  simply sets the "valid" bit in the drive's CDS

See Also: AH=52h,AX=5F08h"DOS"