I like to start troubleshooting tape equipment by checking out all the connection variables. In almost all cases this will involve SCSI connections for the library and drives including drives that are addressed as LUNs behind the library target address.
Make sure that termination is applied correctly (check to see if the library or any of its drives have internal termination active so you don't double terminate. Make sure there are no duplicate addresses on the bus.
If you are trying to share it between two hosts or are trying to use redundant connections, you need to check with the library vendor to see if and how it is supported and any special requirements.
A SCSI bus utility that scans the bus and gathers SCSI inquiry data can be a lot of help. For each address on the bus, report whatever is attached (including LUNs) and what the firmware level is for each target:LUN ID.
Double check to see that the firmware level of your library devices and host adapter are supported by the software you are using for backup.
If it is a Windows machine, you may need to unload and reload SCSI drivers too.
Best of luck,
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This was first published in May 2003