How Does an SD Card Work?

By Sarah Morse

MicroSD cards measure 11 by 15 millimeters and can hold 2TB of data.
i David Paul Morris/Getty Images News/Getty Images

Secure Digital cards, a type of flash memory, were developed by SanDisk, Matsushita, and Toshiba as a potential replacement for the memory stick. Rugged, small and light, they work best for devices that require high storage capacity and portability. As portable electronic devices continue to come with increased functionality, SD cards also continue to improve in speed and storage capacity.


SD cards connect to a device with pins that match a port on the device. Each card has a microcontroller that communicates with the device, bringing data from the host to flash storage components called NAND, or Not And, chips. The microcontroller only activates when when actively saving or retrieving data, reducing the amount of power needed to support the card.

SD Cards have no moving parts, which means that if they are bumped or dropped they are much less likely to break or lose data. The NAND storage is resilient and does not wear easily; data can be written and rewritten thousands of times in the lifetime of the card.


SD cards fall into a number of classifications and categories. Regular SD cards hold about 2GB of data and come in standard, micro and mini sizes. Secure Digital High Capacity cards can hold from 4GB to 32GB of data and also come in standard, micro and mini sizes. Secure Digital Xtended Capacity cards can hold from 32GB to 2TB and come in standard and micro sizes. The device documentation specifies which size and type of card is necessary. Devices are backward compatible, meaning that if they ask for an SDXC card they can use an SDHC card or an SD card.

Manufacturers use a variety of flash memory technologies with varying speeds to create cards. To differentiate between them, the SD Association assigned classes to these speeds of 2, 4, 6 and 10. Each class represents the minimum write speed of the card in megabytes per second. These classes matter most for those wishing to record and save real-time video.


Due to their small size and high storage capacity, SD cards are used in many portable electronic devices. Game systems, digital cameras and video recorders often require a full-size SD card. Smaller devices, like mobile phones and MP3 players most often use a microSD card. An increasing number of computers contain SD card reader slots; if a computer doesn't, a reader can be connected by USB, FireWire, PC card and other ways.

Card Security

Some SD cards have a physical switch on the left side to turn on "write protection." If the switch is turned on, the card can only be read -- no data can be written or overwritten. This protects data from accidental loss. Host devices can designate a password for SD cards or command that they be read-only for extra protection.