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 AH=59h)
Note:   simply sets the "valid" bit in the drive's CDS
SeeAlso: AH=52h,AX=5F08h"DOS"

.