A SSD (solid state drive) is a type of faster hard drive with no moving parts. Wii U supports saving and booting digital copies of games on external drives. Using an SSD results in the fastest load times out of all methods (internal included).
You need a SSD, an external case for the SSD (plus a specific USB 'y cable' if your case does not have its own power source) and a digital copy of MK8. If you find a used SSD, total investment can be about $50.
Controversial, but it's allowed.
Why do you say you NEED a case for the SSD?
"Two of the Wii U's USB ports (since only one port wouldnt give enough power to the drive)"
Interesting. Is that the same for the smash adaptor, too? Where did you find this tidbit out btw?
That's annoying that it doesn't have enough power with just one. Is the Wii U's electrical output weak or something? any idea why?
Much trial and error taught me that. Yeah the USB ports on the Wii U aren't strong enough individually to power the SSD by themselves, so the Y cable basically allows you to draw power from both ports.
SSDs, like hard drives, generally need external cases in order to be used with a USB cable. When a hard drive is in your PC, it uses a SATA cable to connect to the motherboard. Wii U only has external USB ports though, so the case basically allows it to be compatible with USB.