External HDD for Wii U

so my wii u's 32gb is basically all used up atm, and i'm thinking about buying a hdd so i have more storage space. i looked up on nintendo's website for what kind of hdd i should get, and it said it should be one with an external power source, but my dad thinks we could just use one that doesnt need it. so i was wondering, are they actually compatible, and are there any kinds of risks to using them as opposed to the ones with external power sources?

ones with external hard drives perform better, from what i've read

also there's less risk of lost data if it has an external power source

idk but running games from a flash drive works fine for the old wii

don't take my word for it, the guys on the nintendo forums and phone lines are actually extremely helpful for this sort of thing