It doesn't matter what you did before, my 64GB Micro SD Card just borked recently and it was because the phone decided to break it. These things do just happen and it's not usually because of a file or folder.
"NintenDON'T sue me please haha"
late but, everything else was working i was playing ssbm before i launched the game actually and it worked just fine.(note it was on the same exact usb.)