You determine the write-protect status of a disk by passing an IOExtTD to
the device with TD_PROTSTATUS 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_PROTSTATUS;
   BeginIO((struct IORequest *)DiskIO);

TD_PROTSTATUS returns the write-protect status in io_Actual. The value
will be zero if the disk is not write-protected and nonzero if the disk is
write-protected.