Hi all, leaving this post here for future reference, in case anyone needs it. My Wii U was giving issues when launching into Aroma to the lines of 'The SD Card cannot be written to'. This caused the entire system to freeze in my case. You will read plenty of tales online about 'blowing into the slot' or using a piece of paper. But for me what fixed it permanently was bridging 2 pads on the SD card reader slot itself.

For this fix you need to almost fully disassemble the console. I used this handy guide from iFixit. Note, I found that on step 13, you don't actually have to remove the CPU/GPU heat sink, you can just leave it in place and maneuver the EMI shielding around it. (Be very carefull with the antenna cables!!)

When you have the motherboard itself visible, you need to bridge (aka. put some solder connecting these pads) the WP1 and WP2 pads. See the images of my console below for reference. Be carefull not to accidentally bridge any other pins. You can verify this if you want to with a multimeter after.

Before:

After:

This fixed any read-only related issues.

PS. For any Linux users who are coming up against all the online posts suggesting "guiformat" program, the SD association also has an official tool for Linux. It worked for me, KDE Partition manager didn't format it properly for the Wii U's tastes. The SD tool can be found here

Happy modding!