a few checks here:
1. Did you turn on file-sharing on for the WiFi connection on that pc?
2. If you did turn it on, does the NAS respond when you go directly to the ip-adress of the nas. "\\ip-of-nas"
3. What firmware do you use on the router
4. What format do you use on the usb hdd?
5. Did you turn on the samba server on the router?