Target SD Card: one of the Memory Cards Compatible With 3DS, such as a 32GB SD Card. For New 3DS, check MicroSD Cards Compatible With New Nintendo 3DS.

Nintendo officially only supports memory cards up to 32GB -- no matter whether you're using SD or microSD cards and which 3DS iteration you own.

Backing up the old SD card data If you're upgrading from an older 3DS (or just moving to a bigger SD card), you're going to need to back up the data from your old SD card.