Drives keep going DOWN

Have you attempted to move a tape into the drive manually using robtest ? Does robtest show a tape in the drive already ? While it could be something as simple as NBU having the wrong robotic definition for the drive, it could also be a calibration issue with the library, a tape stuck in the drive, someone loaded too many tapes into the library & the drive cannot unload the existing tape to make room for your new tape (no slots available), or something else hardware-related. The diagnostic error message would make me lean towards a hardware problem until proven otherwise but robtest is usually a good place to start your troubleshooting.