NAME
        TD_CHANGESTATE -- check if a disk is currently in a drive.

   FUNCTION
        This command checks to see if there is currently a disk in a drive.

   IO REQUEST INPUT
        io_Device       preset by the call to opendevice()
        io_Unit         preset by the call to opendevice()
        io_Command      TD_CHANGESTATE
        io_Flags        0 or IOF_QUICK

   IO REQUEST RESULT
        io_Error - 0 for success, or an error code as defined in
                   <devices/trackdisk.h>
        io_Actual - if io_Error is 0, this tells you whether a disk is in
                    the drive. 0 means there is a disk, while anything else
                    indicates there is no disk.