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Stands for "Secure Digital." SD is a type of memory card used to store data in portable electronic devices. Examples include digital cameras, video recorders, smartphones, and portable music players. SD cards are considered removable storage (instead of internal or external storage), since they can be inserted and removed from a compatible device.

The first SD cards were introduced in 1999. They used almost the same form factor as the existing MultiMediaCard (MMC) format, but were slightly thicker (2.1 mm vs 1.4 mm). The dimensions of a standard SD card are:

  • 24 mm wide
  • 32 mm tall
  • 2.1 mm thick

Each SD card has a slanted upper-right corner to ensure the card can only be inserted one way. The left side of a card has a physical slider that prevents the card from being written (read-only) when moved to the LOCK position. The "secure" part of "Secure Digital" refers to its built-in DRM protection technology. The SD format supports Content Protection for Recordable Media (CPRM), which prevents protected content from being read from another storage device.

Since its introduction in 1999, the SD card format has gone through several iterations. Below are different versions of SD, listed with their storage capacity and maximum data transfer rate.

  1. SD - 2 GB capacity, 12.5 MB/sec
  2. SDHC - 32 GB capacity, 25 MB/sec
  3. SDXC - 2 TB capacity, 312 MB/sec
  4. SDUC - 128 TB capacity, 985 MB/sec

Another version of SD, called microSD, was introduced in 2005. microSD cards are much smaller than SD cards, with dimensions of 11 mm x 15 mm x 1mm. Small devices like GoPros and smartphones have microSD slots to reduce the overall device size as much as possible. microSD cards can be used in standard SD card slots using a microSD-to-SD adapter.

Updated: April 26, 2019

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