The Backup function in Roon is solely for the Roon database; there is no built-in backup function for the music files themselves.
There is afeature request for this, but nothing has been announced by Roon Labs thus far.
Most of us are using third-party solutions to backup the music files over the network.
You say that you have not found a way of seeing your SSD drive in your Nucleus? If you’ve got a PC running Windows, then it should be visible in the Windows File Explorer, e.g. for my NUC running ROCK:
