NAME
    CMD_STOP -- pause current and queued I/O requests (immediate)

FUNCTION
    CMD_STOP pauses all queued requests for the unit, and tries to
    pause the current I/O request.  The only commands that will
    be subsequently allowed to be performed are immediate I/O
    requests, which include those to start, flush, and finish the
    I/O after the stop command.

IO REQUEST
    io_Message      mn_ReplyPort set if quick I/O is not possible
    io_Device       preset by the call to opendevice
    io_Command      CMD_STOP
    io_Flags        IOB_QUICK set if quick I/O is possible