Yeah, it shows up in DU as well.
It's odd, the checkbox for showing external disks in Finder's sidebar was blue, but instead of a check it had a dash. I'm guessing that meant it was set to only display certain drives, which is odd, because I never changed the setting and, like I said, I've used this drive on this computer. Anyway, I changed it back and now there's no problem. Perhaps I inadvertently changed the setting... or my computer is becoming self-aware and is messing with me.
Hm.
Well, thanks, guys!