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.