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.