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Secure Digital (SD) Cards

Secure Digital (SD) cards are a form of non-volatile memory used for storage in portable devices such as digital cameras, mobile phones, MP3 players, and other mobile electronics.

SD cards come in multiple sizes. The original size, still simply called SD, is roughly the size of a postage stamp. The most infrequently used size, MiniSD, is roughly the size of a human thumbnail. MicroSD, the most popular size for smartphones and other small electronics, is roughly the size of a dime.

The format was created in 1999 by a joint venture between SanDisk, Matsushita, and Toshiba–many consumers mistakenly believe the SD in SD Card stands for SanDisk–and has undergone several revisions since then. The original storage format SDSC (usually labeled simply SD) had a storage capability of 1MB to 4GB. The next iteration, SDHC was capable of storing up to 32GB. The most recent iteration, SDXC, can store up to 2TB. New readers are backwards compatible, but older devices (such as digital cameras manufactured before the release of SDHC) are often stuck using older SD card technology.

SD labels provide additional information beyond indicating the generation of the card (SD/SDHC/SDXC) and the size (such as 4GB). The labels also carry a small class number partially enclosed by a circle such as 2 or 10. The class number directly corresponds to the minimum sustained write speed for the card in MB/sec. Thus a class 10 SDHC card will write, at minimum, at 10 MB/sec where as a Class 2 card has a lower write threshold of 2 MB/sec.

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