I've removed it, and I'm trying again, but just wondered if anyone else had this problem? I thought bittorrent was checksumming chunks as it goes and this sort of thing shouldn't happen.
Transmission has an option 'Verify Local Data'. It will re-check the hashes.
Yea, I've just never had any reason to need it before :-).
Turns out my actual problem is apparently with shoddy USB on my motherboard. When I started poking around I found a few other backed up files that wouldn't verify. Putting the same USB drive on a different computer worked, so now I have a PCIE usb card on order to see if that will work better...