Spinners. Been formatted as APFS for at least 2, maybe 3 years with no issues hooked to the former main (Intel iMac) machine.
Having worked through countless possibilities, the only one left that I think I can try is to reformat with Monterey. However, I've been reluctant to try that because I don't want to potentially lose the drives if it randomly ejects during the formatting process. I don't trust it to stay connected long enough to reformat.
Box is an OWC dual bay, Hardware-selected RAID, set to RAID 0. I think it was a process of selecting RAID-0 on the box, install 2 drives and then use Disk Utlity to format it. Been super stable for a couple of years with none of this unexpected eject stuff. Now I would not bet much on a connection of even 3-4 hours... unless I take it and same cable back to an Intel Mac and then it is as stable as it has been.