WITH money scarce, you would have thought public schools would be grateful to receive any computers lying unused around the office or home. Not so. On average, it costs them around $400 to bring a geriatric computer up to snuff for the classroom.

Better to have your obsolete machines recycled commercially, or to find a charity that will refurbish them for a worthwhile cause. Both will probably charge a small fee for handling the disposal. But that's better than consigning them to a landfill, where toxic chemicals used in their manufacture can leach into the water table.

Before computers became throwaway items, machines that were no longer powerful enough to do the job or broken beyond repair were kept as backups or cannibalised for spares. Your correspondent has a handful of superannuated computers lying around the home. The sprightliest of the bunch—a 400-megahertz Pentium II that came loaded with Windows NT4.0—has found a new lease on life as a Linux server. That aside, it's time for the rest to go.

But how to prepare a computer for reincarnation, if not final rights? Obviously, the first thing to do is remove all your old personal files from the hard drive. People can be surprisingly negligent about such things. Over a third of the hard drives resold on eBay are said to contain personal data such as credit and medical records.

Alas, wiping a computer clean is not as easy as it may seem. Just deleting the personal files and emptying the recycle bin is next to useless. The delete function only removes file names from a directory list and makes the sectors that the files occupy on the hard drive available for future use. In the meantime, the files continue to exist and can be retrieved by basic recovery programs such as Norton UnErase or freebies like Brian Kato's elegant Restoration. The same happens when you empty the recycle bin.

How about reformatting the hard drive and re-installing the operating system? That doesn't do much good either. Again, the data aren't actually deleted, but remain there waiting to be plucked by some nosey parker seeking your identity and your bank account.

Actually, there is no fool-proof way to delete data stored in a computer. Government agencies effectively take a sledgehammer to the hard drive itself when disposing of highly classified information. The Pentagon even removes the platters from their hard drives and minces them in a mechanical shredder.

If physical destruction isn't practical, defence officials recommend that you use “multiple block overwrites”. Overwriting a 100-gigabyte drive (modest by today's standards) at least three times and verifying the result will take several days. But even then, the process can miss lots of blocks of hard-drive space that have been reassigned for other uses.

Experts say there's only one way to drive a stake through a hard drive's heart. First, run the low-level destruction routine called Secure Erase that's hidden inside most hard drives these days (but never mentioned in the manuals because it's such a loaded gun). Second, blast the drive with a humongous magnetic field. Third, grind the platters to dust.

Flickr/George Hotelling

Still hoarding information

But even this may be not enough. Scientists at the Centre for Magnetic Recording Research at the University of California, San Diego, have put shards of ground-up platters under a scanning magneto-resistive microscope and reconstructed traces of the original data. And despite the crew's valiant efforts to destroy all the hard drives aboard the EP3 spy plane after an emergency landing on Hainan island in 2001, to this day no one knows whether the Chinese were able to reconstruct the aircraft's highly sensitive information. You have to assume they did.

That said, what allows most cloak-and-dagger folks to sleep easy at night is knowing that, though fragments of confidential data can always be recovered from hard drives, it takes an inordinate amount of time and effort to achieve anything worthwhile. That's what spooks mean when they talk about “security by exotic time-consuming technology”.

In the real world, the answer is to use a protocol that, while far from perfect, makes reconstruction too tedious a chore to undertake, and yet remains simple enough to use in the first place. Generally, that means buying a proprietary program such as PC Inspector's E-Maxx, Iolo's DriveScrubber, Webroot's Window Washer, Symantec's Norton Utilities and Jiiva's SuperScrubber (for Macintosh). For about $30, any of these will wipe a typical hard drive to military triple-overwrite standards within a couple of hours.

The truly paranoid can always download a more user-friendly version of Secure Erase, called HDDerase, that's available for free from the utility's original author at the Centre for Magnetic Recording Research. But remember, there's no recovery from HDDerase. It overwrites every single track on a hard drive, including directories, bad blocks, partly overwritten blocks—you name it.

Unfortunately, that means the operating system gets obliterated as well. So if you want to donate a computer, sanitised but in working condition, then use something like Windows Washer to remove your confidential files but not the operating system. The charity will thank you profusely, especially if you remember to include the original disks, the manuals and, above all, the licence numbers of the proprietary software installed. And you can rest easy knowing that your personal files are safe—or at least relatively so.