I've been having the same problem the last two days and finally solved it! It's true what they're saying, and it makes sense but sadly you NEED to be using a 32gb sd card or less since FAT32 fs only natively runs on volumes of space up to 32gb.
(Anything larger I believe uses a different hardware layout in the case of flash memory, and creates virtual volumes of larger FAT32 to artificially tells computers they use FAT32 file systems.)
And I've tested 3 different cards to prove it, luckily we had an old 32gb lying around in my dad's room:
Tested for the homebrew method only,
a 2gb worked,
32gb worked,
and 64gb didn't work.
They say use a 2gb for the hackless method (which is really just the anybody-can-do method, a softmod-less workaround) because vanilla Brawl only reads 2gb cards or smaller for some reason. However proj+ is a 2gb installation so best to just get a 2gb-32gb card period if you want this mod.
Some people reformatted their sd card and it works, one person cleared everything on their card except for the project+ files and it worked, and some people I heard in different places downloaded a file from some cord server ((involving gecko launcher I think)) and that solved it for them but none of the links I found for it were valid so that will forever be a mystery. Don't know which cards they used but I do know for the people who switched sd cards to a range between 2-32 they all usually got through.
Anyway this is where I went wrong and didn't notice: PLEASE PLEASE PLEASE make sure, if you want to keep your files that you currently have for other homebrew applications, you MUST make sure every other Brawl mod file is cleared from the sd card! I didn't recognize it but I went to every length and back trying did format allocation sizes, different cards, everything to keep my files and play it, but a simple text file in the root folder, a remnant from Project M called gameconfig.txt was all it took to stop the game from loading each and every time.
my recommendation, try either emptying the drive or placing everything else in another folder on your computer than loading ONLY the Project+ files directly from the zip folder from the website onto your 2gb-32gb sd card and it should work smoothly on a homebrew.
For a no-homebrew I have no experience but I will say try making sure your sd card is read first by the wii at the home screen, and by Brawl itself wherever it prompts you to save to an sd card.
