Which type of SD card should I buy?

Using an SD card (Secure Digital)  is a great way to store data and move data between a myriad of different devices.  For Instance, taken photographs on your digital camera, and then inserting the disc into your laptop to edit the files, then taken the disc directly to your specialised printer to be printed. SD cards are also a great way to increase the amount of storage on your mobile phone.

The use of SD cards has become increasingly popular, and the technology is this sector is continually improving. Unfortunately, as is the case with most technological advancements, it is easy for the casual user to quickly lose touch, get confused or even daunted by the choices available.

When buying an SD card there are 4 important criteria you should  take into consideration your use case criteria, physical size requirements,  storage capacity, and the speed at which it can write data,

SDHC Speed Classes1. Use Case

The first thing you need to consider is, what is going to be primary use of the SD card. For instance, are you only going to use it as a means of moving files and documents between differing devices? Most devices available on the market today come with built in SD card ports i.e. Laptops, smartphones, tablets, digital cameras, game consoles and raspberry pi, often enabling you to view the SD card as extensible storage capacity. However, there is a catch not all SD card ports are created equally. Although this does not matter when it comes to reading data from a disc, it does make a big difference when writing data to a disc. An example is the difference is requirement when writing word processing document, as opposed to High Definiton Video.

Using a class 10 card in a device that can only optimally write to a class 4 disc, is not going to improve performance but it will not have any adverse effect, conversely using a class 4 card in a device that optimally designed to work with a class 10 disc, can have a severe degradation in performance and your device may not operate as expected.

Before purchasing a SD card, ensure you review the capabilities of all your devices you’re potentially going to use the card on, to determine the most efficient class standard.

2. Physical size

SDHC card SizesThere are now 3 different sizes of SD cards. SD, miniSD & microSD.

The original format started with cards measuring 32 x 24mm, which was very small for the time. These are still common in digital cameras, audio recorders and similar products. Smartphone makers wanted smaller cards. This resulted in the miniSD format, measuring 21.5 x 20mm, and then microSD cards, measuring 11 x 15mm.

You can use a miniSD or microSD card in an SD card slot by plugging it into an SD-sized adapter, most manufacturers of SD cards provide adaptors or sell cards are sold with them. It is needless to point out that you can’t fit an SD card into a miniSD or microSD slot, but I’ll do it anyway.

3. Data Storage

There are 3 different types of storage capabilities in SD cards ; SD, SDHC (High Capacity) , SDXC (extended capacity).

When SD cards were first introduced in August 1999 they could hold up to 2GB of data, which was a huge amount at the time, but most cards stored a lot less. In 2006 the industry introduced high capacity SDHC cards which could store up to 32GB using FAT-32.   in 2009, SDXC cards were released which are able to store up to 2TB using Microsoft’s proprietary exFAT (Extended File Allocation Table) format.

With the release of Mac OS X 10.6.5 (Snow Leopard), Apple started supporting exFAT,  making it the defacto standard for shared hard drives or for the transfer of very large data files.   This paves the way for future SD cards to use exFAT to store data upto and potentially beyond 512TB.

Generally 8, 16 and 32GB SD cards are big enough for most purposes. It is possible that some users may choose 64 or 128GB SD cards to expand capacity on their smartphones or MP3 players.  However, before purchasing these size cards, it’s important to check that your device can support it.  The Samsung Galaxy S5, is one of the first phones to support 128GB microSD cards.

4. Data writing speeds
The SD Card Association has different classes of cards, and produced a bunch of mandatory logos with the class number inside a big C. However, Class 2 is 2MBps, Class 4 is 4MBps, Class 6 is 6MBps, and Class 10 is 10MBps or faster — sometimes much faster. For this reason, faster cards are often marked with the speed in large letters, such as 45MB/s or 90MB/s.

Consulting the specifications of your device, you should be able to find out how fast it can write data to an SD card. As long as your cards can handle that data rate, you should generally be ok.

A point to note is that class rating shows a minimum speed, and not the actual speed. A good Class 2 card may work faster than a Class 6 or even a Class 10 card.

Buying SD Cards

You can buy any card that your device supports from any manufacturer. However, I would recommend buying SD cards by respected brands, such as Panasonic, Verbatim, Kingston, SanDisk, Samsung, Toshiba and Sony. Panasonic invented the SD format, and developed it with SanDisk and Toshiba. I also warn you against using any “cheaper” Chinese imports, as quality and reliability of these are extremely questionable.

Gary Woodfine

Freelance Full Stack Developer at threenine.co.uk
Helps businesses by improving their technical proficiencies and eliminating waste from the software development pipelines.

A unique background as business owner, marketing, software development and business development ensures that he can offer the optimum business consultancy services across a wide spectrum of business challenges.
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