Comment by actionfromafar

Yes, it could work. You would have to "disconnect" A and B for files to magically show up, because the drivers on A and B don't expect the filesystem to change "under their feet". It doesn't monitor the disk blocks for changes and rebuild directory indexes, file tables etc.

If you want a truly "automatic" solution, your magic USB device would have to pretend to be a network device to both A and B, perhaps with DHCP or ipv6 autoconfig, and export NFS and/or Samba to both ends.