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.