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 AH=59h)
Note:   simply clears the "valid" bit in the drive's CDS
SeeAlso: AH=52h,AX=5F07h"DOS"

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