"User has opted for target storage from group XXX but device SmartDisk belongs to group '(null)'" (4042530)

Customer was performing a Phase 1 backup to SmartDisk (SD) and Phase 2 to a tape library that used the "Week" Group Label, but the Phase 2 backup was hanging with the following error messages:

1] Warning 2011/11/22 05:00:32 849 Media backup01.travelbox.local No suitable device for job 849 (instance 318) - request held in queue

With sub text:
User has opted for target storage from group 'Week' but device 'SmartDisk - backup01.travelbox.local' belongs to group '(null)'

2] Error 2011/11/22 08:18:19 849 Jobs backup01.travelbox.local Job Status: Duplicate Aborted

Performed a backup straight to SD without labels and completed successfully.
I then ran a backup straight to the tape library without any labels. This job hung with the same error as above.
I checked Media Management and there were enough media i.e. Blank media to deal with the backup, but noticed that there wasn’t a drive added to the tape library.
I checked the Operating System that the tape library and drive could be seen with cat /proc/scsi/scsi and were visible, so the issue wasn't there.