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(Secure Digital Memory Card) A flash memory card that provides storage for digital cameras, cellphones and PDAs. Introduced in 1999 by Panasonic, Toshiba and SanDisk, cards up to 64GB are available with much higher capacities expected in the future (see SDXC below). By 2007, it became the most popular format for point-and-shoot, digital cameras.

     SD Cards use the same 32x24mm form factor as the earlier MultiMediaCard (MMC), but are slightly thicker (2.1mm vs. 1.4mm), and SD Card readers accept both formats.

Fast Storage
Although SD Cards support encryption and content protection (the "Secure" in SD), they have been mostly used for regular storage due to their small size and fast transfer rate (10 and 20 MB/sec). SD uses NAND flash technology (see flash memory).

miniSD and microSD Cards
Miniature variations of the SD Card were introduced with dramatically smaller footprints: the miniSD in 2003 and the microSD in 2005, the latter previously called "TransFlash." Electrically, as well as software compatible, miniSD and microSD cards fit into a regular SD slot via an adapter (see below).

SD High Capacity (SDHC)
SD High Capacity (SDHC) cards provide storage up to 32GB. Based on the SD Card Association 2.0 specification, SDHC cards do not work in SD devices, because SD uses the FAT16 file system, and SDHC uses FAT32. However, SDHC devices support both SDHC and SD Cards. See also Video HD.

SD Extended Capacity (SDXC)
Introduced in 2009, SDXC cards raise the maximum capacity to two terabytes (2TB) by using the exFAT file system. However, the first SDXC cards on the market had 32GB and were compatible with SDHC (SD 2.0 specification and FAT32). See Eye-Fi, gruvi card, SDIO card and MultiMediaCard.




The SD Lineup
The tiny microSD is only 15x11x1mm, compared to the miniSD at 21.5x20x1.4mm and the full-size SD Card at 32x24x2.1mm. The "gruvi" card at top left is a copy protected microSD for paid content (see gruvi card). (Image courtesy of SanDisk Corporation, www.sandisk.com)





MicroSD Adapters
With adapters, smaller cards can plug into larger slots. These microSD adapters let microSD cards plug into miniSD, full-size SD and even USB sockets.







SD Cards Vs. MultiMediaCards
SD Cards are similar to MultiMediaCards, but thicker (2.1mm vs. 1.4mm). They also have indents on their sides, two more pins for data and a locking mechanism used for file protection.