You determine the presence of a disk in a drive by passing an ioexttd to
the device with td_changestate set in io_command.  for quick i/o, you must
set io_Flags to IOF_QUICK.

   DiskIO->iotd_Req.io_Flags = IOF_QUICK;
   DiskIO->iotd_Req.io_Command = TD_CHANGESTATE;
   BeginIO((struct IORequest *)DiskIO);

td_changestate returns the presence indicator of a disk in io_actual. the
value returned will be zero if a disk is currently in the drive and
nonzero if the drive has no disk.