I tried following this with mkfs to format the card on Linux, but it didn’t work–the 3DS couldn’t find the card. Then I borrowed a friend’s Windows PC and used MiniTool Partition Wizard, first quick erasing the card (it wasn’t very quick) and then creating a new FAT32 partition as described above, and that worked great! I noticed the partition setup was a little peculiar before wiping it though, so I suspect that if I had removed and created a partition on the SD card with fdisk before formatting it with mkfs it would’ve worked.