OK, lets talk about NAS
With the B2's NAS function switched on, you can see the B2's "music" folder from a PC or a Mac.
Thus from a Windows PC you can use "File Explorer" to edit, rename, cut/or/copy, and paste the folders and files on he B2, just as if they were on your PC.
Thus, when we say "Backing up with NAS", we mean, from a Windows PC or Mac, copying the content of the B2 "music" folder to a backup folder on a disk or device attached directly to the PC.
In your case, if you also have a NAS (Synology diskstation), and you have this open on your PC or Mac at the same time as the B2's NAS, you could copy the content of the B2 and paste to a folder on your Synology NAS.
There IS NO special Brennan backup function in relation to its NAS capability, the work to achieve a backup is all done from the PC or Mac.
Be advised however that the B2's central computer is just a small, low powered Raspberry Pi and copying files to and from it over ethernet (NAS connection) can be slow ESPECIALLY if you are using WiFi and the WiFi strength is
poor. For a large CD collection the copy could take hours ad hours. AND this will be exacerbated if you are copying to another NAS (as this copy has to through the controlling PC or Mac before going back out to the Synology).
It is also important NOT TO USE THE B2 while the copy is happening B2s do not like doing 2 things at once.
Also remember if you copy stuff to the B2's NAS you need to do a "Disk Scan" after the copy ends to ensure the changes to the B2's content are picked up by the B2's music database. If this is not done the new stuff may not appear in the B2 UI.
You should then press the stop/play button until "saving" appears and let this complete before powering off.
I hope this has helped/clarified.
Fred