Hi Craig,
Looks like 'Fast' catting is using storage-media based cats and 'slow catting' is rebuilding the cats. I would assume in a real-world disaster, we wouldnt have the option of fast-catting anyway.
I've been deleting the cat folder every operation. I'm currently trying a new Full backup that only spans a single tape for simplicity.
The previous tape i was using was a monthly tape for jan so it was written in jan 2012 then jan 2013 there. It could be possible that there have been changes to the backup structure or something in those 12 months.
Currently it's catting and showing a byte count and speed. Previously on the iffy tape - it would occasionally make a lot of sound as it moved through the tape and scanned dirs/files but would not show any byte values so fingers crossed!