Secure Digital (SD) cards are a form of flash memory used to store data from many modern digital devices. They differ from other storage media in that they can be written to thousands of times and do not require power to retain their contents. This makes them an ideal choice for electronics such as digital cameras, which have strict power requirements and need media that can be written to multiple times. In contrast to traditional hard drives, SD cards also contain no moving parts, so they are significantly more resilient to accidental bumps and falls. This has contributed to their use in MP3 players, which are often subject to shock.
To make use of an SD card, your electronics must first be capable of reading the cards. Electronics such as digital cameras and cell phones often contain an SD slot but may not be modified to accept the cards if they do not. Many computers include memory card slots for SD and other memory cards, and you can purchase USB memory card "readers" to enable your computer to use the cards otherwise. Some electronics cannot read all sizes of SD cards, so make sure to read your owner's manual.
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Data in an SD card is stored on a series of electronic components called NAND chips. These chips allow data to be written and stored on the SD card. As the chips have no moving parts, data can be transferred from the cards quickly, far exceeding the speeds available to CD or hard-drive media. The NAND chips in an SD card do not wear out easily, so data can be written to them thousands of times over their lifetime, making SD cards resilient and long-lasting in use.
To prevent the accidental loss of data, many SD cards come equipped with a "lock" for the data contained on the card. This is accomplished via a small switch, which, if enabled, does not allow new data to be written or old data to be overwritten on the card. Because this protection is built into the SD card itself, you do not need to worry about whether your electronics will respect this lock. When data no longer needs to be protected, the switch can be toggled, unlocking the card for use.