Secure Digital

SD Short for Secure Digital card, a non-volatile (flash, solid-state) memory card used in digital cameras, phones and other mobile devices developed in joint by SanDisk, Panasonic, and Toshiba. These memory cards are much smaller than the rival CompactFlash cards although they don't have such high rates of data transfer. Standard SD cards have an official maximum of 2GB, although can technically store up to 4GB. SDHC(Secure Digital High-Capacity) have a maximum capacity of 32BG, and SDXC (Secure Digital eXtended Capacity) can hold up to 2TB. Of note, SDHC and SDXC may not work in older devices. SD cards have a standard footprint of 32 x 42 x 2.1mm. the SD format also contains several variations based on the same standards but with different dimensions and several small variations in technical details. Examples include miniSD and microSD.

Formats
SD cards are based on the MultiMediaCard (MMC) format, although contain a different format. Simple passive adapters can be used to use SD cards with CompactFlash and PC card slots. Likewise, some SD cards come with an integrated USB connector for using the card as a USB Flash Drive.

Some SD card have an option write-protect tab. When on, the card cannot be written to. Likewise, some companies have released software or media on SD card that are likely DRM encoded and write-protected.

File System
Most SD cards are formatted with the FAT for FAT32 file system with an MBR partition. Some smaller capacity cards (under 4GB) can be used with the FAT16 system, although this also limits the maximum file size greatly. SDXC cards use the exFAT (FAT64) file system. Dragmentation will slow down a card over time, although defragmentation may slowly wear out the device. A sector can be written to roughly 100,000 times before it wears out and fails.

Transfer
Basic SD cards all support three standard modes at up to 25MHz for standard cards and 50MHz for high-speed cards:

One-bit SD mode
 * Separate channels for command and data information
 * Proprietary transfer format.

Four-bit SD mode
 * extra pins are used while others are reassigned

SPI mode
 * Serial Peripheral Interface Bus: a simplified SD protocol meant for use with microcontrollers.
 * optional mode on microSD cards.

In addition, UHS-I is meant for SDHC and SDXC cards for compatible devices. UHS-I
 * transfer speed of up to 104MB/sec

Some SD devices support DRM using the Cryptomeria cipher (C2), although this is relatively uncommon, and the specification is only available to license holders.

Legal
The SD format is less open than CompactFlash and USB flash memory drives, which are open standards. The SD standard requires licensing, royalties, and documentation for commercial production of SD cards.

Speed
SD cards are typically given one or two rating formats: either a "Speed Class Rating" or an "x rating".

Speed class ratings are defined via an integer such as "Class 1", "Class 2", etc. One class number is approximate to a transfer speed of 1MB/s, so a Class 8 SD card would have a transfer rate of 8MB/sec.

The "x rating" system is equivalent to 1.2Mbit/s, for a 40x SD card would have a transfer rate of about 48Mbit/s (6MB/s)

Availability

 * Sparkfun
 * Memory Bites
 * My Memory