Check it once, check it twice, check it three times
Sometimes working with computers (and machines in general) can just drive you batty. We had a PC connected to a microfilm reader that was no longer talking to each other. We could’t figure out why. We tried the normal things like reinstalling drivers, changing cables, SCSI cards, even different PCs. Well, we were on the right track with the SCSI cable, but it turns out we had multiple bad SCSI cables so even though we checked with multiple cables, the backup cables were bad. I guess the moral of the story is to make sure when you test with backup cables (or any equipment) that they actually work with your setup.