ETD_CLEAR and CMD_CLEAR mark the track buffer as invalid, forcing a reread
of the disk on the next operation. ETD_UPDATE or CMD_UPDATE would be used
to force data out to the disk before turning the motor off. ETD_CLEAR or
CMD_CLEAR is usually used after having locked out the trackdisk device via
the use of the disk resource, when you wish to prevent the track from
being updated, or when you wish to force the track to be re-read.
ETD_CLEAR or CMD_CLEAR will not do an update, nor will an update command
do a clear.

You clear the track buffer by passing an IOExtTD to the device with
CMD_CLEAR or ETD_CLEAR set in io_Command.  For ETD_CLEAR, you must also
set iotd_Count to the current diskchange number.

   DiskIO->iotd_Req.io_Command = TD_CLEAR;
   DoIO((struct IORequest *)DiskIO);

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