Was using an internal hdd in an enclosure before, started having problems(couldn't download directly to the hdd, had to download to the Wii U memory, then copy it over, eventually that stopped working with large files. Then I started getting the same problems I'm having now). So I bought a 500GB Buffalo external hard drive(Y cable), it was working great for a while, then I started getting occasional error's, now I'm getting them all the time, and I'm lucky if I can even get the Wii U to recognize it when I turn it on. I haven't tried the front usb ports yet.
I think it's a problem with my Wii U though, it has other issues as well with loading, startup takes a while, longer than it should, or used to. I guess I'll look into Nintendo's support and see if there's anything that can be done, but I'm worried they're just going to blame the HDD's.
The old one I was using works fine with my PC now btw, and there's absolutely nothing wrong with the Buffalo one, it hasn't been moved at all.