I have just got a usb Media Adaptor and usb drive, the USB Adaptor is a NAS and has Upnp AV (did say on the box works with Xbox), the problem is my xbox see the drive but can't find any file (Pic, Music, Vid). Itune's finds the drive straight way and play fine.
I know the file stream ok, as they will play from my PC fine.
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I'd try the support forums for the exact product you have, as they are more likely to know what settings need to be enabled to share to Xbox.
Best of luck though
on google you will have all the things you need and want, have fun:)