[jan-jaap]

I also attached a SCSI Terminator to the unused external SCSI port, thinking that perhaps the terminator on my internal Ribbon cable was bad.

Ah, you see, that's where you were mistaken.

SCSI is a chain, which has to be terminated at both ends. The SCSI controller is a device on the chain, like a disk. The chain can go external (to an external CDROM, tape, disks or scanner), but at the end it must be terminated. If you do not attach external devices, you must install the terminator.

The terminator on the ribbon cable is the other end of the chain.

So this works:
  TRM <-> internal disk <-> controller <-> TRM

And this works:
  TRM <-> internal disk 1 <-> internal disk 2 <-> controller <-> external CDROM <-> TRM

But this is asking for trouble
  TRM <-> internal disk <-> controller <-> ...

And this is not going to work either:
  TRM <-> internal disk <-> controller <-> external CDROM <-> ...

Sometimes you can get away with an unterminated bus, but really you shouldn't do this. Also, active terminators are better than passive.