Secure Digital or flash memory (non volatile) memory card format used in portable devices, including digital cameras and handheld computers. SD cards are based on the older MultiMediaCard (MMC) format, but most 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 offer a write-protect switch on the side. Digital rights management features are available but are little used. SD cards generally measure 32 mm × 24 mm × 2.1 mm, but can be as thin as 1.4 mm, just like MMC cards (see below).
There are different speed grades available which use the same measurement system as CD-ROMs: a multiple of 150 kB/s. Basic cards transfer data up to 6 times as fast (900 kB/s) as the standard CD-ROM speed. High-speed cards are made with higher data transfer rates like 66x (10 MB/s), and high-end cards have speeds of 133x or higher. Some digital cameras require high-speed cards to record video smoothly or capture multiple still photographs in rapid succession. As of December 2005, most devices comply to the SD card specification 1.01, with maximum speed of 66x. Higher speeds of up to 133x are defined by specification 1.1.
Devices with SD slots can use the thinner MMC cards, but the standard SD cards will not fit into the thinner MMC slots. SD cards can be used directly in CompactFlash or PC Card slots with an adapter. MiniSD and MicroSD cards can be used directly in SD slots with a physical interface adapter. There are some SD cards with a USB connector for dual-purpose use, and there are card readers which allow SD cards to be accessed via many connectivity ports such as USB, FireWire, and the common parallel port. SD cards can also be accessed via a floppy disk drive with a FlashPath adapter.