(Secure Digital Memory Card) The most popular flash memory card for digital camera and other mobile storage. Introduced in 1999 by Panasonic, Toshiba and SanDisk, cards up to 128GB are available with huge capacities planned for the future (see SDXC below).
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, which started out at 10 MB/sec and has steadily increased. SD uses NAND flash technology (see flash memory).
Smaller Cards: miniSD and microSD
SD Cards were introduced with smaller footprints that are electrically and software compatible with SD Cards and fit into regular SD slots via an adapter (see below). The miniSD was introduced in 2003, followed by microSD two years later (originally called "TransFlash").
SD High Capacity (SDHC)
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. SDHC cards also come in miniSDHC and microSDHC formats. See also Video HD.
SD Extended Capacity (SDXC)
Introduced in 2009, and available up to 128GB, SDXC cards raise the future maximum capacity to two terabytes (2TB) by using the exFAT file system. SDXC cards are compatibile with SDHC. SDXC cards also come in the microSDXC format. See Eye-Fi, gruvi card, SDIO card and MultiMediaCard.
UHS-I and UHS-II
In 2010, the Ultra High Speed (UHS-I) interface was introduced for SDHC and SDXC cards at 104 Mbytes/sec. In 2011, UHS-II tripled the rate to 312 MB/sec to support reading and writing high-resolution 3D HD content.
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
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 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.
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