I am not sure how well the rule is working. For whatever reason I could not access my FTP server this morning after the port forwarding was supposedly active. Had to finagle with the settings to get it back on line.
It my have something to do with the fact that if your Public IP changes when the FTP port forwarding isn't active the information from the dynamic DNS doesn't get correctly updated.
Using Parental Controls and blocking Internet access to the FTP server didn't work. It disables Internet access, but not FTP transfers.
I will play around with some settings and see if I can get the on off of the FTP function to work smoothly using software. If not I will plug the shared drive's power supply into a lamp timer and have absolute control. Not great for the drive's longevity but absolute security.