SCSI is a true black art. There are so many possible pitfalls that it would take forever to go through them here. The obvious ones are to check device numbers, termination and the order in which the devices are chained (faster devices first). Failing that, put on a black robe, sacrifice a goat on a moonless night and recite
http://www.faqs.org/faqs/scsi-faq/
Hope this helps. Never used SCSI without some kind of grief....