RZ25s are SCSI 2, but not (AFAIK) Fast SCSI-2.   I don't know about
RZ28s, though - they're late enough in the production cycle that they
probably are Fast.

One other thing that can cause trouble is that Fast SCSI-2 devices
sometimes work if they're close to the CPU, but fail when they're out
on the SCSI chain.   Believe it or not, getting better SCSI cables and
using active termination can clear up this sort of problem.

			       _MelloN_