If you don't care about the explanation, just skip to the last paragraph. Otherwise, read on: Awhile back, it was discovered that Wii U updates would fail if you had a patched vIOS on your vWii in any of the used slots (so it's okay to use slots 200+). The update would be downloaded, but would fail to install. How it determines a non-vanilla IOS is unknown at the moment; it could be an md5/sha1 check, or maybe it simply checks if the ticket to see if it is fakesigned. Either way, it's a problem. Making a NAND backup is a must, and it is helpful in some ways. But not in this one. When you create a wad from a NAND dump (usually with ShowMiiWads, but there are other programs as well), it's fakesigned (thanks to Cyan for pointing that out to me earlier). This wouldn't be an issue on a Wii, but the Wii U is bitchy. The HackMii installer is too, and refuses to use any IOS that is fakesigned. If you want truly clean wads, you need to dump them directly from your console. You can do this with Yet Another BlueDump Mod. Go to System Titles and dump each IOS individually to a wad. When prompted to fakesign the ticket or TMD, say No to both. It's a tedious process, but it could save a lot of trouble in the future.