If your Belkin N300 router can install Shibby successful and if your Belkin has usb port then you can do following step;
1. normally TomatoUSB can detect USB NTFS Harddisk and mount it automatically. Make sure it mount in bottom of USB Support page. If not check Core USB Suport and NTFS was ticked.
2. at NAS / File Sharing page, File Sharing is Yes. If you want authentication, setup username/password
once done, you can access NAS as a server at //[your Belkin ip] use username/password in 2.
the problem is I'm not sure the Operating System of Raspberry Pi can access share drive or not but it should be Linux right? then it should be fine.