This is a master post with links to all the guides, documentation and files that have been published online for repairing the Wii U. If anyone has links to guides or other resources that they would like added to the list, make a post with the links. Each section is credited to the original creator/owner.
General Information
MLC Checker log can be interpreted using the following information:
-30041 is MEDIA ERROR
-3001B is DATA CORRUPTION ERROR
Using this log you can check & verify if an eMMC is bad. If you see many 30041 errors, the eMMC is bad.
GaryOderNichts Links
UDPIH Exploit to boot recovery menu (https://github.com/GaryOderNichts/udpih)
This is the UDPIH Exploit that allows almost all the work to be done on the Wii U.
UDPIH: USB Host Stack exploit + Recovery Menu (https://gbatemp.net/threads/udpih-usb-host-stack-exploit-recovery-menu.613369/)
Thread about the USB Host Stack Exploit + Recovery Menu. Useful video showing when to plugin the Raspberry Pico to run the exploit.
Original Wii U Recovery Menu (https://github.com/GaryOderNichts/recovery_menu)
This is the original recovery menu used for the Wii U. There are many forks of this that add additional features and functions.
Wii U Ident Tool (https://github.com/GaryOderNichts/WiiUIdent/commit/a2f6893cc04cee9b4398252fea4d5366521b4850)
This tool is useful for identifying the manufacture date of the eMMC based on its CID.
SDIO Links
Ultimate Wii U troubleshooting Guide (https://gbatemp.net/threads/ultimate-wii-u-troubleshooting-guide-system-memory-error-160-0103-stuck-wii-u-screen-stuck-factory-reset-black-screen-after-stuck-update.642339/)
This guide has a full list of problems a Wii U can experience, with detailed information to identify the problem and solutions to those problems.
How to Upgrade/Rebuild Wii U Internal Memory MLC (https://gbatemp.net/threads/how-to-upgrading-rebuilding-wii-u-internal-memory-mlc.636309/)
This guide gives a step by step instructions on how to rebuild and/or upgrade the Wii U MLC.
How to set up ISFShax (https://gbatemp.net/threads/how-to-set-up-isfshax.642258/)
Guide for installing ISFShax, a tool that has similar functions to defuse, but requires no soldering. Also is instrumental for repairing Wii U with failed or failing eMMC.
Wii U RedNAND Install Guide (https://gbatemp.net/threads/fixing-system-memory-error-160-0103-failing-emmc-without-soldering-using-rednand-with-isfshax.642268/)
Guide for installing ISFShax and RedNAND. This is simliar to a NAND-AID but requires no soldering of components.
GitHub Repository (https://github.com/jan-hofmeier?tab=repositories)
SDIO Github. The tools used in his guides are found here and also are the tools I have used to repair many Wii U.
V10lator Links
Using NAND-AID to repair a broken eMMC (https://gbatemp.net/threads/using-nand-aid-to-repair-a-broken-emmc-fix-160-0103-system-memory-error.636361/)
V10later thread talking about how to use a NAND-AID to repair a broken eMMC. Very useful information can be found here, especially about how the Wii U internal storage works.
NUSspli (https://github.com/V10lator/NUSspli)
Tool used with Homebrew to install content from the Nintendo Update Servers.
Additional Information
Wii U Factory Reset Log (https://pastebin.com/aZbcFU9T)
Explains how the factory reset works and in what order are files deleted. Something interesting is that the logs are deleted during a factory reset after the MLC data. However often times the MLC data is corrupt and it causes the factory reset to hang, but the logs will be deleted even though it hangs.
Pico Defuse Tool (https://github.com/shinyquagsire23/wii_u_modchip/tree/main/pico_defuse)
Defuse is a powerful tool that can be used to help diagnose/repair Wii U that nothing else works on. This does require significant soldering and a Raspberry Pi Pico to use.
Decaf Emu (https://github.com/decaf-emu/decaf-emu/blob/6feb1be1db3938e6da2d4a65fc0a7a8599fc8dd6/src/libdecaf/src/ios/fs/ios_fs_enum.h#L152)
A list of error codes and what they mean. This is part of the project known as Decaf Emu, which is researching Wii U Emulation.
Tiramisu (https://wiiu.hacks.guide/#/introduction)
Homebrew guide to install Tiramisu custom firmware on a Wii U
Wii U Title Database (https://wiiubrew.org/wiki/Title_database)
This is a database with all the Titles for the Wii U. It allows identifying what is corrupt if you plan on backing up and restoring an MLC that has corrupt data.
Wii U File System Tools (https://github.com/koolkdev/wfs-tools)
A set of tools to be used for various tasks dealing with the Wii U File System. The most notable tool in this lot is the wfs-extract tool which can extract data from an MLC dump file. Additional tools include: wfs-rencryptor, wfx-fuse and wfs-file-injector