I have a Wd passport drive hooked directly to the router. But it doesn't show up anywhere in Xbmc nfs, smb, zero config. And I have an nas my book live that shows up no issue. So not sure what I did wrong with the passport.
In using Xbmc Gotham 13.1 on a fire TV.
Does your router act as a NAS? I somewhat doubt that...
No but it's supports putting a USB drive on it. Why does it need to be nas?
To expose your USB drive via smb, nfs, upnp,... A router doesn't do that.
Check your router's doc to see what the USB port is actually used for.
What model of router is it?
http://www.tp-link.us/article/?id=253
Reading about it, sounds like it supports FTP and UPnP (maybe UPnP. It mentions "media server" functionality, which typically means UPnP/DLNA).
EDIT: it also sounds like it supports SMB.