you must go into the bios and select the usb stick as a boot device and disable the internal USB device, if not it will default to the internal boot USB. QNAP are the same way. Also your external USB stick may have multiple volumes, you may need to try vol2, not just Vol1.
i am so ready to throw my ReadyNAS 516 out the window with its ancient OS. But with a xenon processor and 16 GB of ram, i cannot bear to....