I have seen this problem a few times and here is a list of things that
caused it;

SSO environment and a NON-NBU host was resetting the drive (cron job running
ioscan HP)
A non-NBU application on same host resetting drive during backups. 
Bad Fibre/SCSI bridge, UFSsump, UFSrestore and tar jobs all worked fine.
Lose SCSI cable on the Fibre/SCSI bridge.
Bad or wrong hba driver (had to back rev, vendor support loves to tell you
to get the latest and greatest but sometimes that is part of the problem :-)

I'm leery of saying it's not NBU software related (it is software after all)
but from my experience this problem has always been pinned down to the data
path.

G

Gary Andresen
Impossible Happens, Plan Ahead
Pacific Northwest Data Inc. 
Tel: 503.701.5185
Fax: 503.692.3910
gary.andresen AT pnwdata DOT com
www.pnwdata.com