It’s because if the external IP address changes, the external IP address for the FTP service does not. Presently it needs to be changed. Not a problem if you have a static IP address. It is a small issue if the public IP address of the router is prone to change.
http://community.wd.com/t/for-ftp-service-the-nas-to-automatically-set-report-external-ip-in-pasv-mode-when-external-ip-changes/193772
That Sounds like a good idea.
Hope you get good support on this idea.
