Identify the cables with a marker to be quite sure. Try better and shorter cables. When it works, begin adding the other drives, one at a time. Normally, you should be able to find out the culprit. Think that it may not be the last of the string, but the one in front.
Some users say that the scanner is always best at the end.
Others say the contrary. With a scanner, test the quality of the SCSI bus running a scan on a color paper. Check in your favorite program that you don't have scratches, pits and other problems. Launch a verify on the cartridge to see whether there are bad spots on the medium. You can also do a low-level format, but this destroys all the data, as you know.