Function 4405h (68-5)    IOCTL: Write to Block Device

    Write a control string to a block device.

       On entry:      AH         44h
                      AL         05h
                      BL         Drive number (0=default, 1=A, etc.)
                      CX         Number of bytes to write
                      DS:DX      Data or buffer

       Returns:       AX         Number of bytes written
                                 Error code   If CF is set

       Error codes:   1          Device doesn't support control strings
                      5          Drive is invalid

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    This subfunction writes CX bytes of data to the block device's control
    channel from either DS:DX or from memory pointed to by DS:DX.  How
    data is written is defined by the device driver.  Data can be written
    only if the device can process control strings.