The latest update is unlikely to have caused this issue. Since you can see the drive from the Mac, which I assume is your core, it is more likely a permissions problem (note the error message.)
I suggest you go to System Preferences and check Roon has access in the Security / Privacy settings.
If Roon has Full Disk Access, then I try removing this and re-adding. In addition, if you create a folder on the Mac with an album, and add this to Roon, does it work.
Ben here with the support team, I wanted to check in on this thread and see if you were still running into permission issues with your external backup device? @Martin_Webster is steering you in the right direction with the next steps.
Hi Ben,
The issue has been resolved by giving the external drive further permissions that it previously had. Must be something to do with the recent Apple macOS update.
Thanks @Martin_Webster for your assistance.
Cheers
Chris