When connected to the Internet, computer(#B) doesn't exist (from the internet point of view), so the copied data is totally immune.I use my network card to connect directly to another computer(#B) for backup, and then I change the network cable and connect to internet with the same network card. Does this make my shared folders (with read/write permissions) more vulnerable to attacks?
you're already intervening by recabling. You leave P/F sharing active for backups
and disable when connected to the Internet.