I booted with the Western Digital Diagnostics floppy that I keep handy, and when I did a quick scan it said it found a problem and asked me if I wanted it to fix it. I did. Then I did the full scan, and it didn’t find any problems. So I booted into Linux single user mode. Couldn’t figure out how to fsck the drives that way, so I booted with the rescue disk. It wouldn’t let me unmount the drives that way, so I booted back into single user mode so I could do a “shutdown -rF now”, which forced it to do a fsck when it booted again. It didn’t find any problems. So now I’m continuing to run my backup process every two hours, and keeping an eye on it. 36 hours later, still no more disk errors.