What Are SD Memory Cards and How Do You Buy Them?

Views 1 Like Comments Comment
Like if this guide is helpful
What Are SD Memory Cards and How Do You Buy Them?

The digital world is making a bigger and bigger impact on each of our lives every day. With this new wave of digital devices comes a need for reliable, portable, and compatible storage systems to serve these devices. The Secure Digital (SD) card is just such a system, providing storage solutions to an impressively diverse range of devices. Everything from digital picture frames to mobile phones make use of SD cards, so it is likely that any serious gadget fanatic will want to purchase one of these cards at some point. The information in the guide below will act as a starting point to familiarise the reader with the most important technical specifications to look out for when choosing an SD card, allowing further research to be conducted in order to find the best deal.

Physical Size

It may seem obvious, but paying attention to the physical size of a device's SD card slot is a simple way to save a lot of time and effort grappling with mismatched products. The table below provides information on the three different physical SD card designs available.



Number of Pins

Typical Uses




Digital cameras




Digital cameras and some mobile phones




Mobile phones

In general, the newer the card design, the smaller it will be, although companies still manufacture larger sizes of card to cater for the staggering variety of devices which utilise these cards. It should be noted, however, that smaller SD cards can be placed into specially designed adapters with the correctly orientated pins to facilitate their use in devices with standard size slots.

Memory Card Family

SD memory cards are available in four families. Devices which conform to the newer specifications will be backwards compatible, supporting each of the previous families of card, but older devices may not support families which have been produced since the device was manufactured. The four SD card families available are described in the table below.

SD card families


Standard Capacity (SDSC)

SDSC cards were the first to be designed, and they are supported by any device which is capable of supporting the newer types of card.

High Capacity (SDHC)

SDHC cards can be utilised only by devices designed to the specifications of their interface. In terms of hardware, SDHC cards are identical to SDSC cards; the main difference lies in the storage system implemented on these cards, as well as the FAT32 formatting system. These changes mean that SDHC cards are able to support up to 32gb of storage space. The updates to the Card-Specific Data (CSD) register of these cards means that older devices may not be able to support them, although this problem can often be resolved with an update to the firmware of the device.

eXtended Capacity (SDXC)

SDXC cards can be utilised only by devices designed to the specifications of their interface. SDXC cards can accommodate a storage capacity of up to 2TB, and are largely compatible in any SDHC enabled devices. Some compatibility issues do arise, though, due to the newer exFAT formatting system employed by these cards. Some users choose to format an SDXC card with the FAT32 file system to increase its range of compatibility with older SDHC devices.

Input/Output (SDIO)

SDIO cards can only be used by devices which are designed to act as input/output controllers. SDIO adapters can be used to add devices such as GPS transceivers and miniature cameras to PDAs or tablets with SDIO capabilities. Although the USB interface is a much more common method of connecting peripheral devices, SDIO cards are still incredibly important as a mechanism of interface for some applications.

Read and Write Speed

The method which SD cards use to write new data to the card makes the read/write speed of these cards much less than other commonly used forms of storage. While the early SD cards used the 'x' system comparable to that found on optical discs to denote write speeds (a designation which denotes the maximum possible write speed of the medium under ideal conditions), the more modern cards use a Speed Class Rating to represent the minimum guaranteed write speed of the card which bears the rating. The speed associated with each SD card class is shown in the table below:

Speed Class Rating

Minimum Speed (Mb/s)

Class 2


Class 4


Class 6


Class 10


Class 2 cards are recommended for recording standard video files, Class 2 and 6 for HD videos, and Class 10 for Full HD videos, or writing HD still photos in quick succession.

Potential Problems with SD Cards

Any newer device which is able to support one of the more modern classes of SD card will also support each of the previous families of card available. Placing a new SD card in an older device may not always work, though. These compatibility issues can be caused by a number of factors such as a card with a greater storage capacity than the host device is designed to accommodate, or a file system which the host device is not programmed to recognise. Some of these problems may be circumvented by formatting the card with one of the older file systems, or by updating the firmware of the host device, thus conferring upon it the ability to deal with cards of a greater storage capacity. Because of these potential computability issues, it is important to check the specifications of any device before committing to purchasing an SD card. These specifications may be found in the instruction manual for the device, or by doing a quick search online.

Buying SD Memory Cards on eBay

Some of the best deals on SD cards can be found on eBay. Searching for 'SD card' on the eBay homepage provides an unsatisfactorily broad set of results from across multiple categories, so screening for the results found only in a particular category is the ideal way to refine the search. Since SD cards are so widely used, this section will give two approaches: one for camera SD cards, and one for phone SD cards. To find an SD card for a camera, begin by selecting Cameras & Photography under Electronics & Technology, followed by Camera & Photo Accessories. Next, select Memory Cards from the pane on the left of the page. The check-boxes on the left now allow selection of the specific family of SD card, such as SDXC. A search for 64GB, for example, after selecting this check-box results in a highly refined list of cards of exactly the specification required. The search categories can be refined even further by picking the manufacturer or price range required. SD cards for phones can be found by beginning once again on the eBay homepage. Select Mobile Phones & Communication under Electronics & Technology, and pick Mobile Phone Accessories from the pane on the left. Select Memory Cards from the list on the left of the page, and then choose the type of card required for the phone in question. For example, after choosing MicroSDHC, searching for 32GB yields results specific to the search term, and of exactly the type required.


With such a variety of devices making the most of the cheap storage available in the form of an SD card, it is important to know the ways to get the best deal on a gadget of this type, and to have an idea of the important specifications to look out for when making a choice. With the information provided in this guide, any user should have all the tools required to make an informed decision about the type of SD card required, and should have no problem finding the best deals on these items.

Have something to share, create your own guide... Write a guide
Explore more guides