Thanks for all the advice.

I have re-formatted my drive (and yes, it's on the internal interface), 
giving RiscOS 1.92 GB. bb_riscbsd reported exactly the numbers I was 
expecting this time (not negative!) and unixfs seems happy. Supposing it 
does have trouble over 4161 cylinders, this leaves me with nearly 80 MB 
of the BSD partition I should be able to write to safely.

So, the sets coppied over all right (no need for floppy swapping today!) 
and all is going very nicely! (Well, _mostly_ !)

Cheers,

Simon M.