My clients on my network could access FTP servers behind our ISA 06 firewall but they could not create or delete anything on that FTP. I checked to make sure the permissions on the FTP were setup correctly. I narrowed it down to ISA blocking Incoming FTP traffic coming back from the FTP servers. I did some searching and there are many issues out on this. Others issues vary slightly but I did find something that worked for me. I setup the below rule on ISA.
