Based on the results...that evidence would lead me to believe there was a 60/40 probability there was an issue with the drive over the DVR itself.
Well, I guess it may be an issue of semantics. From DirecTV's perspective, there is
an issue with the DVR, because the internal drive is bad. From their perspective, it's not considered a user serviceable part. So the DVR, as a unit, is "bad", and should technically be replaced. However, because I didn't want to hassle with getting a replacement, I simply attached an external drive. Which worked fine until "the glitch", which also, based on the results, caused issues for a disproportionately high percentage of other external drive users.
While it's probably too early to proclaim definitively that the drive is actually fine, preliminary results would suggest that there's actually nothing wrong with the drive: The DVR diagnostics repeatedly said there were no errors, yet it wouldn't boot with the drive. Western Digital diagnostics say there's nothing wrong with the drive. And after wiping the drive and re-attaching it to the DVR, it is (so far) working just fine.
So the results would lead ME to believe that "the glitch" imposed a data error on the drive that the DVR was simply unable to remove. Once it was removed with external tools, the DVR seems happy with the drive. I'm not sure why would think there's a 60/40 chance the drive was the problem.
Friends don't let friends buy PPV.