The mrd_unload(3mrd) function is a specialized interface to the
    SCSI Move Medium command (or DSA equivalent). For the robot
    specified by robot_name, the routine will attempt to move the
    cartridge in the specified drive to the specified slot. Element
    addresses are zero based.

    The robot will be opened and the arguments to the function will
    be verified to make sure they are safe and appropriate. The drive
    and slot address will be verified they are within the valid range
    of those elements on the robot.

    The volume_tag argument can be used to perform cartridge volume
    tag verification before the move. If the cartridge volume tag at
    the port doesn't match that specified by this argument, then mrd_
    unload(3mrd) will fail with the status MRD_STATUS_CART_INVALID.
    If volume_tag argument is a NULL pointer, an empty string or
    used on a robot without vision support this argument is silently
    ignored and the volume tag check will not be made.

    The DLT libraries (TL82X and TL81X families) require the host
    issue a SCSI Unload command before a cartridge may be removed
    from the drive. The function mrd_unload(3mrd), does not offer
    this feature. Thus, the calling program must do this itself. The
    example below shows how this can be done on Digital UNIX.