Secure digital (SD) memory cards are flash memory cards which meet the standards of the Secure Digital Association, a group of manufacturers which sets and certifies SD cards. Typically, secure digital memory cards can store between one and four gigabytes of data, and are designed for use in portable devices like cameras. They are what is known as nonvolatile memory cards, because they do not need to be connected to a power source to retain their stored data.
The secure digital technology was introduced in 1999, and quickly became a widely-adopted standard for memory cards. Several manufacturers got together to establish the SD standard and to found the SD Association, recognizing the value of manufacturing flash memory cards to a common standard. Many of these manufacturers also produce devices which work with these cards.
Secure digital memory cards have several features which distinguish them. The first is the option of a locking mechanism, available on most SD cards. When locked, data cannot be written or overwritten, protecting the information on the card from accidents. When unlocked, the card can be overwritten, reformatted, and so forth. The size and shape of secure digital memory cards are also distinctive, with the card being about the size of a stamp with an asymmetric design which is designed to ensure that the card is always inserted correctly. The contacts in the card are also protected so that they are not damaged when the card is handled.
Secure digital memory cards come in a number of different formats. MiniSD and micoSD cards are available, as are secure digital high capacity (SDHC) cards. The main difference between an SD card and an SDHC card is that the high capacity card can carry more than 4 gigabytes. Some older devices designed for SD cards may have trouble reading SDHC cards, but as a general rule, the two can be used interchangeably. People can also access SDXC technology and “extended capacity” cards which have a very high storage capacity.
In addition to being used in various devices, secure digital memory cards can also be read by secure digital card readers, allowing people to pop cards out and connect them to a reader which interfaces with a computer or another device to pull the data off quickly. Many electronics stores sell card readers along with SD cards and other types of memory cards. When purchasing a memory card, it is important to make sure that a card of the right standard is purchased to ensure that it will work. If a device calls for microSD cards, for example, people cannot use SDHC cards with it.