New hard drives are in and seem to be doing fine. Old hard drives are out. I had a new hard drive dock to help with the process, too:

Thermaltake BlacX Duet 5G ST0022U ABS Pl... Station, Black

I had a single-drive eSATA version but felt like upgrading to a dual-drive. This one uses USB 3.0. I didn't actually time it, but I think it took like 4-5 hours per drive. One drive had ~600 gigs on it, and the other had ~800 gigs.