I had issues with an upgrade and the problem was solved here.

https://forums.unraid.net/topic/198248-very-slow-rebuild-after-swapping-in-new-drive/

I put in a new power supply and this fixed the issue. I started the sync a few nights ago and walked away from it until right now.

The sync was completed but now the new drive and another drive shows as unmountable.

I don't know what I need to do to try and fix this without data loss. The array appears to be running normally still.

I have attached smart reports and diagnostics. Thank you.

tower-diagnostics-20260415-0412.zip tower-smart-20260415-0415.zip tower-smart-20260415-0418.zip

As mentioned in the online User Manual you need to check filesystem when you get a drive showing as unmountable.