I decided to reformat it, and it's working normally so far. (Other than the lost recordings! I STILL say it's a software issue, not hardware!) I haven't tried reconnecting the EHD yet though, I want to wait til I don't have anything being recorded in case it decides to crash or something.
A few odd things I noticed before I formatted:
First, I would swear that somewhere in the diagnostics menu, there was a way to see the temperature log, the last fsck time (what I was REALLY interested in!), all sorts of non-obvious codes, etc. I couldn't find it. I tried hitting the Details button, which is where I thought it was, but it just said "OK" or something else stupid like that. (I haven't checked after reformatting -- again, I don't want to disturb anything just yet.)
Second, the satellite number that would be displayed in the lower left corner of the Info screen would sometimes shimmer. Yes, shimmer! It looked like an unstable analog signal, like a VCR recording. Nothing else on the screen would do this, and the satellite number wouldn't do it all the time. Definitely the strangest thing I've ever seen the system do!