NAME
        CMD_UPDATE/ETD_UPDATE -- write out the track buffer if it is dirty.

   FUNCTION
        The trackdisk device does not write data sectors unless it is
        necessary (you request that a different track be used) or until the
        user requests that an update be performed. This improves system speed
        by caching disk operations. These commands ensure that any
        buffered data is flushed out to the disk. If the track buffer has not
        been changed since the track was read in, these commands do nothing.
        ETD_UPDATE command checks for diskchange.

   IO REQUEST INPUT
        io_Device       preset by the call to opendevice()
        io_Unit         preset by the call to opendevice()
        io_Command      CMD_UPDATE or ETD_UPDATE
        io_Flags        0 or IOF_QUICK
        iotd_Count      (ETD_UPDATE only) maximum allowable change counter
                        value.

   IO REQUEST RESULT
        io_Error - 0 for success, or an error code as defined in
                   <devices/trackdisk.h>

   SEE ALSO
        cmd_write