Perhaps your drive cannot go as fast as 30MB/sec?
I've seen SCSI-3 drives which would not go faster than ~10MB/sec with CSPPC SCSI, yet when swapped with a different brand/spec/size SCSI-3 drive under the same chain and cabling, I'd get 30 or more MB/sec.
Not all SCSI-3 drives will play nicely with CSPPC. Most will work, but your mileage and performance may vary.
