You could reset the library to defaults, just remember to save the settings before you do.
Our TL4000 had two drives in it, one LTO6 (HH) and one LTO4 (HH) for our older media.  A while ago the LTO4 drive failed so I removed it, but the TL4000 still is reporting it's presence to the host OS in a 'failed/offline' state.  The front-screen shows an exclamation mark (!) under the drive slot the LTO4 was in.  I've tried rebooting the whole thing but it still won't go away.  I can't put the LTO4 back in because it was causing the whole library to lock up when it was initializing.     It's currently still in the 'annoyance' category as the LTO6 drive still works fine, but the host OS and Backup Software give me an error every time I restart things because the TL4000 still appears to be reporting the failed drive. Any thoughts?     Thanks!
TL4000 - Remove 'ghost' drive