SD memory cards are things we seem to take for granted
I recently saw an item on the BBC technology news pages announcing the launch of a 512gb SD card. These tiny devices, measuring just 32 X 24 X 2.1 millimetres are now the standard storage method for digital cameras – both still and video. Indeed, the same news item mentions that the motivator for developing such a large capacity SD card is the latest “4X” standard for video production. This can require up to 5gb for every minute of video shot.
The price of this new Sandisk card is £490. At first glance this may look expensive, but it’s less than £1 per gigabyte and the price is bound to drop as sales rise. Note that this entire article looks at SD cards – not solid state hard drives (very different animals).
From my own experience of computer support clients who use SD cards at all, it seems most people just accept that they are the storage method in their cameras and don’t use them for anything else. Moreover, if they transfer their images to a computer via a cable (instead of by removing the SD card and sticking it in their computer) then they probably never give the card a moment’s thought.
SD cards do have other uses, though.

2gb Class 2 SD card
Since the card doesn’t obtrude, you can leave it there forever if you want and use it as a backup drive. Used this way, it could save you from disaster in the case of hard drive failure but, it must be admitted, it won’t be any good as a backup if you leave your laptop in a taxi. Oops.

2gb Micro SD card and adaptor
The instructions for my smartphone (a Sony Xperia) specifically say that it can use cards up to 32gb. This probably means that it is limited to the “SDHC” version (which means Secure Digital High Capacity). If it were compatible with the latest standard (SDXC) then it could use the new Sandisk card mentioned above and any bigger cards as they are released – right up to 2tb (this is 2 terabytes, also expressed as 2000gb).
Apart from storage capacity, another factor to bear in mind when buying SD cards is the speed at which they can read and write data. Obviously, faster cards are more expensive. The speed is expressed in a “class” number. Most cards indicate that they are between Class 2 and Class 10 (where the figure represents the number of megabytes per second that can be read or written. The card must be able both to read and write at the stated speed). Even faster cards are UHS 1 and UHS 3 (representing 10mb and 30mb per second respectively). Class number doesn’t matter so much if you’re using the card for still images, but go for a high class number if recording video or if you take still images in “bursts”.

You can store about 20 times as much music on a 32gb SD card as contained in this pile of cassettes
Click this link for an even nerdier look at the humble but hard-working SD card.