do you mean there are magnets in the wii u that let the nand chip fry's when it is open's?
or do you mean that i put magnets on the wii u that i have never done! because i have not opent the wii u yet.
i've had issues like this with old (and fake) cards. my laptop at the time would read them, but other things wouldnt, then other devices would read part of them or have broken blocks etc. buy a new sd card rather than using old random ones, i wouldnt use a micro Sd card for a wii u anyway, standard sd cards are more reliable.