I have an iPod Nano 5 15GB, an iPod Classic 150GB, and a iPod Shuffle 2GB.
The Nano shows in Music, but perpetually says 'Loading' and the 'Sync Settings' are greyed out. It also shows in Finder but perpetually says 'Loading'
The Shuffle shows in Music, but perpetually says 'Loading' and 'Sync Settings' are greyed out. It also shows in Finder but perpetually says 'Loading'
The Classic shows in Music, but perpetually says 'Loading' and the 'Sync Settings' are greyed out. It also shows in Finder but perpetually says 'Loading'
I have restored the Nano on another computer as suggested here and then once back to factory settings, plugged it back into my MacBook Pro 2016.
The Nano now still shows in Music, but perpetually says 'Loading' and the 'Sync Settings' are greyed out. It also shows in Finder but perpetually says 'Loading'. It also no longer has no music on it obviously and no way it would appear to get new music on there.
I am unwilling to repeat this with the Classic and Shuffle as although they don't have any new music at least they are still usable. I regularly update all my devices as I purchase new music and download new music (DJ mixes) every week and rely on Smart Playlists that automatically add new music to playlists.