INT 6F - 10NET v3.3+ - UNMOUNT
        AH = 18h
        AL = local drive number (0=A:)
        BL = type
            00h     disk
            01h-03h LPTn
            04h,05h COMx
Return: CF clear if successful
        CF set on error
            AX = error code (see AH=00h)
SeeAlso: AH=17h

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