Can you please clarify precisely where you created your Backup folder (give the complete filepath)?
You say you created it on the internal storage drive of your Nucleus, but this sounds wrong to me.
A Nucleus has (potentially) two internal storage drives:
- an M.2 SSD used to hold the Roon OS and the Roon software
- a 2.5" drive (SSD or HDD) that is used (exclusively) to hold local music files and folders.
You must not create a Backup folder on either of these drives, or you will get problems.
You can add a USB drive to the Nucleus and this can be used to hold a Backup folder, as well as an additional Watched Folder for your music files.