That doesn't make any sense... the NAND is in your 3DS. It has nothing to do with your SD card.Changing SD card.
I used this guide https://gbatemp.net/threads/how-to-use-a-64gb-micro-sdxc-in-your-sdhc-compliant-flash-cart.335912/, original SD card = same issue. I tried easeUS too but that didnt work either. I'll check around for other apps i guess.
Edit: so i tried other stuff, like a full format of the 8gb with windows and now BOTH* sd card just read "no SD card found" in the 3ds, tell me i'm tired and FAT32 isint the right format please. An old SD card i have lying around, formated in fat32 read as a valid SD card just fine. wtf?
Are these cards MicroSD cards in adapters, or full size SD cards?