I'm confused on how the 2018 files would end up on the 2017 USB drive to get ruined by the /bb folder backup.

Your BB folder, with all the RT subfolders, etc, would need pretty much the whole USB drive, so you should assume the 2017 data is what will be lost in the long run. I don't see how 2018 files could be affected.

If there is a given folder you are concerned about, rename it to 'filename2017'
That way if duplicates would have overwritten themselves before it is possible to retain both versions (labeled) as long as space permits.

Make your sound your own!
.. I do not work here, but the benefits are still awesome