While troubleshooting, I removed the Parity and for whatever reason when I reconnected it, Disk1 tried to rewrite. I'm sure it was user error. Therefore, to my knowledge, the Parity drive is completely wiped... which sucks because Disk1, with the data, is so slow that it'd take >1 month to rebuild.
Thankfully, I backed up the server's contents to a completely separate external HDD. Sounds like disk1 is trash - hopefully WD will replace it under warranty.