Hi. Is it possible to access the USB disk connected to the router from the WAN side via FTP?

On my WL-500gP FTP works only from the LAN side. I then tried to set up a virtuel server, that should route trafic from port 20:21 on WAN to port 20:21 on 192.168.1.1, but it didn't do the trick.

Is it possible at all?

I use original 1.9.7.5 firmware.

Thanks.