As I said, hard to disprove but seems unlikely. Backblaze says 76.7% chance these SMART attributes will be bad on a failed disk. [1] More importantly, I said I'm using RAID-1, so a single bad disk shouldn't be a problem.

Another poster suggested this happens with AFP, which would likely mean a macOS defect. I'm using AFP. I think I'll switch to SMB...

[1] https://www.backblaze.com/blog/what-smart-stats-indicate-har...