Flash memory cards are a handy way to increase storage memory on digital devices of all types. They are especially useful for smartphones with limited built-in storage because image, music, and video files can quickly fill up internal memory.

This article looks at the technology behind various Flash memory cards and focuses on Secure Digital (SD) storage cards—the most popular format. These cards are widely available through computer and camera stores and through a variety of online vendors.

Recent innovations are leading to dramatically increased storage capacities and improved data transfer speeds. But they are also causing some consumer confusion.

A little SD card history

Secure Digital Flash memory cards were first introduced to the market in mid-2000. The SD Card Association (sdcard.org), a non-profit organization and strategic alliance, was formed to develop next-generation semiconductor memory (Flash), set industry standards for SD-based formats, and promote their use.

Well-received worldwide, the SD format is used in a variety of devices today, including digital video and still cameras, MP3 players, portable/handheld computers, automotive multimedia systems, video game consoles, PDAs, and mobile phones. File storage capacities for the original format SD cards ranged from 8 MB to 2 GB in size. As of 2008, newer high-capacity SD (SDHC) cards had capacities that ranged from 4 GB to 32 GB.

Standard SD cards

About the size of a domestic postage stamp, the "standard" SD card was the dominant format used in mobile phones, PDAs, and other digital devices a few years ago. These cards were initially available in 8 MB capacities, but this quickly grew to a maximum of 2 GB. Consumers found the SD card easy to use for storing electronic files and moving them from their digital camera to a PC or a photo-printing kiosk.

On casual examination, an SD card is similar in size to its predecessor—the MultiMedia (memory) Card (MMC). However, many (but not all) SD cards have a small notch on the side housing a write-protect tab, which lets the user lock or "secure" data from being overwritten. Both the standard SD card and newer high-capacity cards have the same physical footprint. The newer cards have an "SDHC" on the front.

Given the greater acceptance of the technology, better card inventory availability, and decreasing production costs, card prices continue to drop. The price a consumer would pay for a 2 GB card today is likely to be the same or less than that of a 16 MB card several years ago.

A Kingston 2 GB microSD card (center), packaged with adapters for a miniSD card slot (right) and an SD card slot (left).

miniSD and microSD cards

The SD format proved to be so popular that as devices became more sophisticated, and their form factors reduced to sleeker sizes, so did the SD card's physical footprint. The miniSD and microSD cards were designed to complement the shrinking profiles of cameras, PDAs, and mobile phones. Produced with similar capacities as standard SD cards, both the miniSD and microSD formats can easily function as standard-size SD cards with the help of adapters that typically ship with smaller cards.