So it turns out it was the HDD, probably the logic board on it or something. I received the replacement HDD from WD and it ended up working. Though it scared the crap out of me because I tried putting it in my Windows system before taking my unRAID server down to test it there, and it would BSOD my Windows machine instantly every time. Whether plugged in directly, or through my USB external drive dock. Regardless, though, I tried it in unRAID, and no issues adding the replacement drive. Glad it wasn't me this time though. I just figured I was doing something wrong.
