Your other option is to use a portable external hard drive which normally gets power through its usb cable rather than an ac adapter.
External hard drive with power supply for wii u.
From the above quote it appears that although the use of a y cable is already good enough you might still encounter a drive requiring more power than even two usb 2 0 ports.
Portable external hard drives.
Due to the wide range of external storage devices usability and performance will vary.
That should see you through the next eshop sale.
This seagate drive is the closest you ll get to nintendo s exact recommendations and you.
Fortunately the wii u supports external hard drives so even if you ve got the standard with its itty bitty 8gb drive you can add up 2tb of space.
In the wii u s case it means you re going to need to purchase a y cable plug the single end into the external and both of the other ends into the two usb ports on the wii u.