I contacted ATTO just after posting my message, and they were able to solve the problem. Here is what they said:
Open the ATTO Configuration Tool. If you do not have the Configuration Tool, please download it from our website. On the left side, click the arrow in front of HOSTS, then LOCALHOSTS, then the ExpressPCIxxxx SCSI adapter. You should now see Channel 1 and Channel 2. Click on the correct SCSI channel. If you are unsure of which channel your device is connected to, please look at the back panel. It should be labeled Channel 1 and Channel 2 accordingly. In the main window, you should now see "Common NVRAM Configuration". Scroll down to "SCSI Target NVRAM Configuration" and change the "Specified Target" to the SCSI ID of your tape device.
Once you have changed the Specified Target, change "Tag Command Queuing" to DISABLED, and change the "Sync Rate" to 160 DT. Click the COMMIT button. If you have a tape library, you should have more than one SCSI ID, and you will need to repeat this process for each said SCSI ID. Remember to click the COMMIT button after each SCSI ID.
After all of the ID's have been modified accordingly, restart your computer. Upon restarting, you should be able to see your devices in the Configuration Tool, as well as System Profiler.
I did all that and now my backups are working fine, except that I discovered that if you have scripted Retrospect to backup automatically and you log out of your user account, it doesn't have access to external SCSI hard drives. That sort of makes sense -- there's no way around that, I suppose?