The increased traffic on the SCSI bus caused the error mentionend above, but only at times of high traffic, during remirror or when doing an ncopy. Turning off read after write verify also let the problem disappear. I have a friend who has had problems with Seagate and Adaptec compatibility. After pulling most of my hair out it was solved by slowing the DMA speed on the SCSI controller. Our backup server was down for almost a week with a bad ultra fast wide SCSI cable to the external RAID array.
Good cables are worth the price.