I had a problem with my N2 connecting to Android File Transfer on my Mac all of the sudden. My issue was from downloading Kies on my computer. For some reason this causes an issue. I tried uninstalling, rebooting, reinstalling everything... nothing works. If you keep Kies on, there is an option to download or update drivers. I did that and it worked fine right after without a reboot. Not sure if this is the same issue, but worth a try.