Sky3DS uses a special format that windows does not recognize. If you want to erase and reuse your micro sd cards you need to use SD Formatter from https://www.sdcard.org/downloads/formatter_4/index.html
I like that games can be fixed after the fact, hate that it's being abused via beta tests... And DLC... I was a 7800 owner back in the day and loved Impossible Mission, turns out I couldn't beat it because it was actually impossible lol