To copy my music library from my iMac to my new B2 I copied all the files to a 240Gb SSD and then imported them using the "Import from USB C" function.
If/when I add files to my iTunes library and want to add those files to the B2 it seems most convenient (for me) to upload them using the WebUI. I can then copy the latest files onto the SSD as a "backup" in case things get lost/deleted or corrupted.
If I had a few albums worth of files to add, the WebUI upload could become too slow. If I copy some "new" files onto the SSD, alongside the existing files, if I attempt an "Import from USB C" operation, will the B2 skip (or ignore) all of the existing files/folders and only import those that don't already exist? It took a number of hours to import all the tracks from the SSD and if any updated "import" is going to take a similar length of time I would be better keeping a small memory stick for "updates", whilst keeping my SSD also updated so that my reserve disk has all files present.