External My Book drive not showing as mounted, but WD Drive Utilities app can see it

About 10 days ago my external "WD My Book for Mac" stopped being available for Time Machine backups.

I've tried rebooting the external drive; the iMac (running Sonoma OS) and pulled the USB cable out and re-seated it.

In going to Disk Utility it is showing as "Not mounted", and I cannot see it in Finder either.

However, when I opened the "WD Drive Utilities" and performed their drive status checks on it, those tests worked fine!

In going back into Disk Utility, the drive shows as greyed out and UNMOUNTED, however, when clicking "Add Volume" or "First Aid" or "Partition", it seems to be able to see it, as it does not throw up any error message.

Any other ideas?

When running First Aid it says "First Aid process has failed. If possible back up the data on this volume. Click Done to continue"