More Wii U FAQ

Q) What is "FSGetMountSource failed"?
A) FSGetMountSource is what tries to mount your SD card so it can read files from it, it needs to be formatted to FAT32 (see footnote for help*) and have the right files in place (see next question). If that checks out, you can try to fix it by saving a QR code from Mii Maker to it and if that doesn't fix it, your SD probably has physical damage.
* FAT32 Formatter I recommend !!!BACKUP ALL DATA BEFORE FORMATTING!!! http://www.ridgecrop.demon.co.uk/index.htm?guiformat.htm
Click on the image and it will download.

Q) What files do I need?
A) The Homebrew Launcher boots by loading ELF files from SD:/wiiu/apps, each in its own folder, the ELF needs to be named the same thing as the folder and you can optionally have a picture and XML for the Homebrew Launcher to display.
When booting itself, it tries to load SD:/wiiu/apps/homebrew_launcher/homebrew_launcher.elf and if it can't find it, it'll just softlock, hold the Power Button and try again.

Q) How do I tell what Smash mods are what? How do I use Sm4shExplorer?
A) If you download a mod from GameBanana, those are the modified "source" files, you need to build them with Sm4shExplorer which is way beyond the scope of this FAQ. Before you try building, make a folder with the TitleID of your Smash version on the root of your SD:
US - 0005000010144F00
EU - 0005000010145000
JP - 0005000010110E00
It'll ask to copy to your SD when you build it. The SDCafiine compatible files should look something like this:
SD:/0005000010144F00/patch/patchlist
SD:/0005000010144F00/patch/resource
SD:/0005000010144F00/patch/data/ with "packed" files inside somewhere which is what Sm4shExplorer builds
If you find a modpack that already has that structure, you don't need to build it or have a dump, you can put it on your SD card and go