There are TWO USB on Arch Pro. The one right next to Push-Button (USB1), another one right next to LAN connector (USB2). The USB1 is for MBED programming, what is the purpose of USB2? Is USB2 function as USB device? Does it have driver for Windows 8.1?
USB2 can be used as a USB device or a Host, it deponds on your program.
Have a look at USBDevice and USBHost of mbed handbook(mbed.org/handbook/Homepage)