Thanks, @noris -
Yes, I know exactly what this is.
My main backup of my Roon database (not music files) is on my iMac. (I always perform backups exclusively from Roon’s own Settings > Backup - sometimes more than once in any one day.)
But every 14 days I - additionally - attach this 64 GB thumb drive formatted exFAT directly to my Nucleus and run a separate backup - again only ever from Roon’s Settings > Backup) as described in this thread.
Although my gut somehow tells me this USB ‘clue’ may be a false lead because I’ve experienced this phenomenon - as have several others on the forums (as I know you know 
) - before I bought my Nucleus last November, I’d of course hope you can investigate further.
For instance: is it possible that Background Audio Analysis was being performed at the moment I ejected the exFAT thumb drive - even though I had waited for the Roon backup to finish completely - and that somehow Background Audio Analysis must rely on external backups to complete?
I get no errors and am able to verify the USB drive in question. Also the male-to-female USB 3 Type A connectors etc.
Thanks again for helping this !