For New 3DS, check MicroSD Cards Compatible With New Nintendo 3DS. A computer with a built-in SD Card reader or an external SD Card drive.

MicroSD Cards with the use of an SD Card Adapter SDXC Memory Cards (Through an update, SDXC cards up to 128GB are now compatible to the 3DS provided it is somehow formatted to fat32 which it ...

Safely ejecting SD cards and USB flash drives is standard practice to avoid potential data loss - at least on the surface, that is.