USB storage attached to a Lumin product shows up on the network as a UPnP media server, not network file system. To manage it you'll have to plug the USB storage into a computer.
Lumin products can play music stored in a computer by simply installing the free MinimServer on the computer.
After reading this I wanted some clarification. I have a USB memory stick attached to my current music player and its software recognizes the files attached. When I read about the U1 mini (and U1) I was assuming you could do the same with it. Insert USB Drive (stick or other form of USB hard drive) into the USB port on back of U1 and the Lumin software would allow the songs to play.
I'd like to validate if playing music via a USB drive connected directly to the U1 mini will work this way. Thx