So I did some more research and it turns out this is a known issue with the MX500 drives:
https://www.reddit.com/r/unRAID/comments/niqvdk/current_pending_sector_is_1_should_i_be_worried/
https://forums.unraid.net/topic/79358-keep-getting-current-pending-sector-is-1-warnings-solved/
https://www.hdsentinel.com/forum/viewtopic.php?f=32&t=12514
From what I'm seeing, it's a firmware bug that's been around for years. I'm just using the drive as Plex storage, so I'm choosing to ignore the warning. I doubt at this point that any data was actually affected but I can't say that for certain.
