Using 2,5" drives is only possible with an Y cable as the Wii U doesn't offer enough voltage on the USB ports where the good old Wii did So a 2,5" drive takes up an extra USB slot (two in sum) You could use an active USB hub though As some of you may already know, certain USB selfpowered external hard drive might not run stably with the Wii U due to lack of power in one USB port If yThe intro screen loads up for a while and then the screen turns black and says "The External USB Hard Drive has been disconnected by the Wii U Please make sure you plug it into a AC Adapter or make sure the Y cable is connected properly Please press the power button for 4 seconds to go back to the main menu" OK Nintendo
Using Usb Storage With The Wii U Guide Nintendo Life