I think I have discovered an issue with writing to tape.
My backup strategy is as follows:
Forever incremental -> NAS
Synthesised full -> Tape.
The issue I have discovered is that if no tape is available, the backup to tape job goes ahead and creates the synthesised VBK file, then finds no free tape. Instead of prompting to insert a tape, the job fails and starts again. Until a tape is inserted this process can loop around infinitely (fail -> create VBK -> fail -> ... etc)