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Secure Digital (SD) is a flash memory memory card format. It is used in portable devices, including digital cameras and handheld computers. SD cards are based on the older Multi Media Card (MMC) format, but most (not all) are physically slightly thicker than MMC cards. They also boast higher data transfer rates, but this is always changing, particularly in light of recent improvements to the MMC standard. Most (not all) offer an optional lock switch on the side to prevent accidental overwriting, and (little-used) DRM features. SD cards generally measure 32 mm by 24 mm by 2.1 mm, but can be as thin as 1.4 mm, just like MMC cards (see below). There are two main types available, ones that run at regular speeds, and high-speed cards that have higher data transfer rates.
User experiences have shown that some brands, makes, and capacities of SD cards seem to work better with the GP2X than others, depending on the firmware level. A detailed list is available at SD card compatibility.