I suppose it could be power supply related. If SSDs draw less current, perhaps they'd work better. The drives in the optical slots are powered by the cord that is attached to the stock optical bay. Maybe it just wasn't designed to run a spinning disk. But, seems like a laser would draw some current as well. And maybe the 4,1 has a beefier power supply than the 3,1.
Anyway, just wanted to let you all know.
I'm happy it's working for others and you, David. It was nice having an extra drive in there (while it lasted).
The one I have is a well-built adaptor, machined out of a piece of aluminum. OWC makes good products.