SD cards are not only for external storage in your camera. They are also used with aptops, select portable scanners, some ebook readers , and old models of game controllers.

Camera experts suggest you Format your SD card in your camera (rather than your computer) to minimize the chances of corruption..

Good Sd cards provide fast data trasfer speeds as well as in-camera speeds and don't costs that much.

The best SD card: 64GB Samsung Pro Plus (very fast daat transfer speeds)

The second best SD card: 64GB SanDisk Extreme PRO (faster than Samsung Pro Plus, a bit pricey)

The best affordable SD card: 64GB Transcend W60MB/s (fast enough to shoot 4K video)