For that one install I did in the other post, I used Mocha.Have you tried using Mocha?
Also, forgot to mention, the USB error I get also shows up when you try to download an uncomplete WUP and WUPINSTALLER still installs it, I know this because the other day I converted Super Mario Galaxy (wich is a Wii game, and Wii games do get installed on my Wii U unlike Wii U games) but the WUP was incomplete, it installed anyways but and the error happened.
I could get the WUP file of that game to be complete after, what I'm trying to say is that the error when a WUP is incomplete is the same I'm getting when installing my Wii U and N64 games.
But, those WUPS are not incomplete, most of the N64 ones I have tried out are convertions or the ones in UtikDownloader, also from sites with isos and that stuff, for the Wii U, they're also from UtikDownloader and in some other cases, iso sites.
So, the errors are the same, but the reasons are unrelated, for the Wii game, the WUP was incomplete, for N64 and WiiU, it is unknown but it is clearly not because the same reason.