Plex + Nvidia Shield + Music on External Hard Drive..How do I?

I got an Nvidia Shield recently, and I have an External hard drive with quite a few songs on it, and what I'm wanting to do is hook up my External HD DIRECTLY to the Shield and have Plex load up my music.. I tried doing it over the internet but after it loaded about 30 artists into my collection I could not load ANYTHING, it would just say "loading" (or whatever it says) and would never load any artists collection, so I'm resorting to having to do it this way... Anyways.. I tried hooking up the hard drive, and loading Plex and setting the Shield as the server, but then nothing happens, it doesn't detect anything. So how can I do this successfully?

Thanks!