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Cyberclinic: 'Will my old data be safe when I sell my computer?'

By Rhodri Marsden

Published 22/08/2007

Even when we delete files we no longer need, they're often not wiped completely
Even when we delete files we no longer need, they're often not wiped completely

The people who write programs strive to give us lots of well-organised menus, but most people – myself included – have no idea what's happening behind the scenes. Sure, we usually remember where we've saved documents. But temporary files can be created automatically and stored in unusual locations, so that we can recover our documents after a crash or power cut.

Even when we delete files we no longer need, they're often not wiped completely. As a result, data that you thought had long-gone may well be still there, and files that you'd deleted by accident can often be recovered using inexpensive software.

This is great for the absent-minded, error-prone person who has accidentally trashed their 80,000-word autobiography. But it also means that secondhand computers can be mines of fascinating data, so it's important to know how to delete it all.

Simply reformatting a hard disk and reinstalling the system software does make a computer appear "as-new", and will usually deter curious new owners from taking a peek at the previous contents. But the Royal Canadian Mounted Police, apparently, recommend a triple overwrite with random data – and there are ways of doing so, without paying a penny.

Steve Bryant writes: "On Macs, when you reinstall OS X, you can either choose a quick format, or a 'zero data' option, which overwrites your entire disk with zeros." PC users, however, will need to get hold of some special software. "There are a few paid-for solutions," says Jon Mackereth, "but there's a free one called Eraser (www., which is available for most versions of Windows."

Be warned: zeroing data takes time, but it can be worthwhile – particularly if you're working high up in the world of espionage. Whatever you do, though, do make sure you've backed up all that precious data before you start.

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