The USB on the back of your Premium Wireless product  is for firmware updates only.

The best way to stream music from a drive is to add a NAS (Network Access Storage) Drive to your network, or connect your USB drive to a device that allows connected drives to be accessed over the network. Most aftermarket routers include a USB port in the back allowing a drive to be connected.

The USB drive should be Windows formatted to either FAT32 or NTFS. Sharing may need to be enabled on your router.