I bought this 3ds pre modded from someone last year around Christmas, so I do not know how old the sd card is, but it has been working fine so far and shows no errors when checked with h2testw.
I opened it up yesterday to see all my games and themes gone. The storage options from the settings reads the sd card as in, but it shows it as empty. It lists some games(but not all with streetpass functionality) in the streetpass communication options weirdly? When checked on a computer the sd card seems normal with no corrupted files, and all data intact. It reads games normally from the game slot in the back.
I have tried reformatting the card, checking the card with h2testw, cleaning the pins with isopropyl alcohol and deleting the "3ds" folder letting the device create a new one on startup.
I maybe stupidly tried to boot to the luma startup screen to see if it was reading the card and lying to me, it worked but when I tried to proceed to the home screen it showed a big ol "exceptional error" screen(I'll post it in the comments). And now anytime I turn it on with the option for booting to the luma startup screen thingy it shows it again(boots normally with it off). I'm kind of lost on what to do next. I have another newer sd card I could use but it is smaller than this one and I'd like to save this one and the save data with it if I could.