I have seen this error a number of times, and there are 2 basic possibilities. The second is that the tape drive is non functioning, and this will require the jukebox be shutdown and the drive replaced. The first occurs after a backup or clone job abends, or a tape label is unreadable.
So here is what I start with:
1. Determine whether there is a tape in the drive. Depending on the model of jukebox, you many be able to see if the drive is loaded.
2. If there isn't a tape in the drive, review the logs in /nsr to determine the tape that caused the failure. I have seen this problem caused by bad tapes more than bad drives.
3. Run an nsrjb -I -E to reinventory the jukebox, the results can be viewed by running a nsrjb afterward.
The listing in the jukebox will appear at the bottom.
4. Enable the drive either from the GUI or from nsradmin.
5. If the listing show a tape in the drive, run the command
nsrjb -u -f /dev/rmt/1mbn to unload the tape. If a tape unloads, you will probably want to eject the tape into the jukebox cap to eliminate the possibility of the tape being used by another drive. Use the nsrjb -w command to move the tape into the cap.
6. Now for the dirty part. If there isn't a tape in the drive, awk through the log, keying off of the drive name, and determine the tape volume that was mounted when the drive reported it's first I/O error, also in the /logs/messages file. You may have to manually place this tape into the bad drive, and power cycle the jukebox to recover the drive [after shutting down Legato...]. This has actually recovered more drives for me than replacing the hardware.