Hi Richard,
        A few Q's for you and a few suggestions. First, is it any job
that runs to those drives? Are you sure it's not the same jobs over and
over? Is there anything unique about the connection to these drives. Are
they attached to a different server than the rest? Has the workload for
that server changed? In almost 10 yrs of supporting backup HW, I've only
seen 3 cases where the drives were actually causing the problem and they
were all manufacturing defects. 
        Outside of that, Dirty drives can often have a higher retry rate
which can slow down backups considerably. This can be verified by
checking the retry counters and error counts in NBU. I would think that
if this was the problem, you would see these drives requesting cleaning
frequently. On this note, do you have automatic cleaning set up? Has NBU
been receiving cleaning requests often? Are you sure the cleaning tape
in your library still has cleans available? 
        Lastly, are you bringing this data across the network? Has the
server where the data resides gotten busier? Has the network gotten
busier? If dragging data across the SAN, has there been an increase in
SAN activity during the backup window? 
        fI hope these questions can point you in the right direction.


Mark Pinder
Systems Engineer
Spectra Logic