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Secure Digital (SD) Cards

A Secure Digital (SD) card is a small non-volatile electronic memory device. Non-volatile is just another way to say it does not require power to retain data stored on it. Simply speaking, an SD card is like a really small cassette tape that can hold photos, music, or other computer files. SD cards come in a wide variety of physical sizes and capacities. There are also several different “classes” available.

SD Card

If you’re unfamiliar with SD cards, this article is for you. SD memory cards are no more complicated than CD’s, DVDs, cassette tapes or vinyl records. With a little bit of explanation, the many benefits of using SD cards are easy to understand.

Secure Digital (SD) memory cards are becoming increasingly common. You may already own a device that uses SD cards. Newer mobile (Cell) phones, digital cameras, portable audio players, PDAs, video cameras and video game consoles use SD cards for data storage. Many computers also have dedicated slots for SD cards.

SD Card Physical Sizes

Currently, SD cards are available in three physical sizes; standard, mini and micro.

Standard SD cards are about 32mm x 24mm. These are the most common SD cards.

Mini SD cards are about 21.5mm x 20mm. These are probably the least common SD cards.

Micro SD cards are the smallest size SD cards, measuring about 15mm x 11mm. Micro SD cards are often used on devices like mobile phones and smaller portable audio players.

Because of the various sizes available, adapter cards have been developed to enable the use of mini and micro SD cards in devices that use standard size SD cards. The adapter cards look just like a standard size SD card, but they have a small slot on the bottom end that a micro SD card fits into.

SD sizes

SD Card Capacities

SD Card CapacityFollowing advancements in data storage technology, SD card manufacturers are continually offering higher capacity cards. The highest capacity cards available just a few years ago are now some of the lowest capacity cards on the market today. The high capacity offered by new SD cards enables users to store and transport more data than CDs, cassette tapes, or floppy disks. The only drawback from the increasing capacities is that devices that use SD cards may only be compatible with SD card capacities that were available when the device was developed. For example, older digital cameras may not be compatible with the newest, highest capacity cards available today. A device’s user manual can provide compatibility specifications.

Sometimes, people are unsure which SD card will best suit their needs. To determine this, first decide which devices the SD card will be used in. Check the owner’s manual for compatibility specifications. Next, determine how much data you’ll be wanting to store. As a rough guide, an average (four minute) song recorded at medium quality (128kbps) uses about 3.75MB of memory. Therefore, more than 500 songs could fit onto a single 2GB SD card. (For more info on data size, please see the section titled, “Understanding data measurements” toward the end of this article.) Finally, you’ll need to choose which class…

SD Card Classes

The easiest way to understand the differences between the various classes is to think of the class number as a speed indication. In this case, the speed we’re referring to is the data write and transfer rate of the SD card. The class rating of an SD card is indicated by a number in the middle of a "C". See the photo below. Currently, there are four common classes available; 2, 4, 6, and 10. These numbers represent how much data in Megabytes per second the card can read or write. Higher class numbers can write more data faster as well as transfer the data to a computer faster. So if your goal is to transfer all the data on a 32GB card to your computer, a higher class SD card will perform the task quicker. Also, if the SD card is used for a device like a new high definition video camera (which records lots of data per second), you’ll need a higher class card to handle the writing of all that data. For general purpose, it is advisable to avoid high capacity cards with low class numbers.

SD Card Class

A few more words used to differentiate SD card classes are: SDSC, SDHC, Ultra and Extreme. SDSC means SD Standard Capacity and refers to cards ranging in capacity from 1MB to 2GB (although there are a few 4GB SDSC cards out there). SDHC means SD High Capacity and refers to cards ranging in capacity from 4GB to 32GB. Ultra is just another word used to advertise SDHC cards. The word extreme is sometimes used to advertise cards with fast data read/write speeds.

Benefits of SD Memory Cards

Store almost anything

SD memory cards are extremely versatile. They perform the same functions as familiar data storage devices like compact discs (CDs) or cassette tapes. However, SD cards can store many types of data on the same card. Photos from a digital camera, music and video collections, documents, computer files etc. can all be stored on the same SD memory card.


SD cards are one of the most durable memory storage formats. The majority of us have experienced either a scratched CD or a worn/stretched cassette tape. These are frustrating experiences that often mean the loss of data (music, movies, etc.). SD cards do not scratch or wear out. Some are advertised as water resistant and all are pretty difficult to really damage.

No Moving Parts

Unlike CDs, cassettes, or hard-drives, SD memory cards do not require moving parts during use. This not only enables devices that use SD cards to be physically smaller, but to also be more durable. In many cases, this lack of moving parts helps lower manufacturing and assembly costs. Another benefit of less moving parts is the conservation of energy. On portable devices, this benefit is very useful as the conserved energy may be used to power extra features or provide longer battery life.


As you’ve already read, SD cards are small! This makes them more portable than CDs, DVDs or cassettes and also means the devices that play back the content can be smaller (more portable). Since you can fit more data on SD cards, you don’t need a special case to carry multiple cards. And because they can store many types of data, you can use an SD card to keep your photos, videos, music, and documents, in one place which can be accessed from a wide variety of play back devices.

SD Card Lock SwitchSD card tips

SD Card Lock Switch

SD cards feature a small sliding lock switch which (when engaged) disables any data transfer or changes (deleting, adding, playing back, etc.).

SD Card Readers

For devices or computers without SD card slots, there are devices called SD card readers available. They usually plug into computers with a USB connector and feature a slot to load the SD card into.

Understanding data measurements (KB, MB, GB)

Data Measurment Table

SD cards are quickly becoming the most common portable data storage format. Considering the benefits offered by SD cards, the increasing gain in popularity is justified. For those of us that grew up with CDs and audio (or video) cassette tapes, SD cards are a welcomed step in the evolution of data storage technology.

For more information about SD cards, please visit the SD Card Association's website.

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