Inventory fails with 'device is not ready' although backup works ok at night

I have a customer who's backup works ok at night but every time I try to inventory the drive, no matter what time, it fails suggesting the device is not ready. They are using BE 11d with an HP Ultrium 3 drive. I have tried everything I can think of to get this working but it is really annoying as it is worrying me that the backup will stop working at night.

 

Any suggestions?