I have managed to test my tape drive with a Promise PCI card and the problem 
doesn't go away, so I'm not so suspicious of the VIA chipset anymore. I 
haven't managed to test the Zip drive yet.

I have sent email to Seagate, and I'm about to start using a DOS box to run 
some test code (DOS rather than NetBSD so that I can fiddle with the hardware 
more easily).

Cheers,
Lloyd