Secure Digital (SD) is a memory card format for use in portable devices, such as mobile phones, digital cameras, GPS navigation devices, and tablet computers. The Secure Digital standard was introduced in 1999 as an evolutionary improvement over the MultiMediaCard (MMC). MMC cards can be used in SD slots.
Secure Digital cards are thicker than MMC, have asymmetrical slots in the sides to prevent inserting it upside down, and the electrical contacts are recessed beneath the surface of the card, protecting them from contact with a user’s fingers. Most Secure Digital cards have a sliding tab that covers a notch in the card to make the card read-only.
The Secure Digital High Capacity (SDHC) format, introduced in 2006 supports cards with capacities up to 32 GB. The Secure Digital eXtended Capacity (SDXC) format, introduced in 2009 supports cards up to 2 TB. SDXC cards are pre-formatted with Microsoft’s proprietary exFAT file system, so SDXC cards are not usable in all devices.
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