They are SATA drives (on individual ports), and the comp has been rock solid for the past 3 months. I've seen one blue screen in all that time.
It's probably more likely to be a heat related issue. It's a desktop case - rare these days, but I wanted that style because originally I was planning to stack 3 of them and that way they'll consume less of a footprint. The mistake I made was not realising that a desktop case has less room overall, which means that you can't simply stack the hard drives like you do with a tower. One is in the HD caddy (with a fan directly blowing onto it), one is in a 5 1/4" drive bay (still has good airflow) and one is on the side of the chassis with only a vent as airflow. That's the drive that failed.
There's no way to add in an extra internal fan near the "hot" drive, but I guess I could mount one on the outside of the case... or just change to a tower case that can house them properly.