What is an SD card? Here's what you need to know about the small memory cards for electronic devices

SD cards, short for Secure Digital cards, are small thumbnail-sized memory storage cards with max capacities ranging up to 128TB.

How to format an SD card and erase all of its data

You can format an SD card using Windows, Mac, and Android devices, and likely your digital camera too.
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How to Use SD Cards Bigger Than 32GB

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. There's no guarantee a larger one will work, though ...