Cyberclinic: Why is buying a memory card so confusing?

"Oh, and while you're there, could you get a memory card for my camera? " Send someone on a shopping expedition with these instructions, and your gofer might get disorientated by jargon such as "dual-voltage MMCplus". This isn't to say removable memory cards are a bad thing; they're one of the most fantastic examples of the distillation and refinement of technology.

Ten years ago, I bought a hulking computer with an awe-inspiring 2GB hard disk; these days you can get 2GB of storage on a card that slips into your phone and is the size of a thumbnail. But what are all the various types? And what's the difference between them?

CompactFlash comes in two types – one slightly thicker than the other – and they're used primarily in professional cameras. They can store huge amounts of data (16GB is not unusual), and it's quick to download your photos from them, and for the camera to store the pictures you take.

MMC stands for MultiMediaCard; the original MMCs came in a standard size or reduced size (RS-MMC), but they each have a modern, faster cousin: MMCplus and MMCmobile, respectively. There's also an even smaller version, the MMCmicro, which is mind-bogglingly minuscule.

SD, or Secure Digital cards, come as either normal, mini or micro; they're based on MMC but are slightly thicker, and have a tab to stop you accidentally deleting your pictures if you don't mean to.

Then you have Sony's Memory Stick (not to be confused with USB drives people carry on key rings) which, again, comes in three sizes; and finally Olympus's xD, which comes in one size only, and is slightly slower and more expensive than its competitors.

There is some compatibility between all these cards. The smaller ones can often be magically turned into their bigger brothers just by using an adapter – which is often free. Other adapters exist to fool slots into accepting cards of a different format, while MMC cards can be used directly in SD slots (but not necessarily the other way round).

PCs are compatible with all of them, with cheap and widely available USB card readers allowing you to transfer easily your pictures, movies and tunes.

Aside from CompactFlash, all these various cards do roughly the same job at roughly the same speed, with little discernible difference between them – indeed, you'd be unlikely to choose a particular sat-nav or MP3 player purely because of its memory card format. So, as long as all these blank media cards remain widely available, the diverse and slightly confusing market is set to continue. You just have to ensure that you get hold of the right one for your gadget – possibly by making that shopping trip yourself.

Diagnosis required

Next week's question comes from Lawrence Cross:

"I'm passing on my old computer to someone else – but how can I be sure that all my passwords and personal information aren't still hiding somewhere on the hard disk?"

Any comments, and new questions for the Cyberclinic, should be emailed to cyberclinic@independent.co.uk