How to securely erase your hard disk

Reformatting your hard disk, or restoring Windows from a recovery partition, will remove any visible trace of your files – but it may not remove the data. All a quick format does is set a marker to indicate files are no longer available, with the contents remaining on the disk until more data is written over the top.

How to securely erase your hard disk

Even if you install a fresh copy of Windows, it will overwrite only a portion of the disk: sensitive data could still be hanging around elsewhere on its platters, and a data-recovery application may be able to bring that data back. That’s why it’s important to perform a secure erase if you’re planning to sell on your PC. It’s unlikely that the buyer will have the expertise or desire to resurrect your personal data, but why take the risk?

Secure deletion software

There are plenty of tools that can wipe a disk securely: we’ll use the free Darik’s Boot and Nuke (DBAN) utility. As the documentation explains, “DBAN is a means of ensuring due diligence in computer recycling, a way of preventing identity theft if you want to sell a computer”. It works by completely overwriting a disk with random data before finally blanking it, so that nothing that was there before can be recovered. Ignore the disclaimer that DBAN offers “no guarantee that data is removed” – it’s more than good enough to secure consumer drives.

DBAN 2.2.7 comes as a small ISO file that you’ll need to burn to a blank CD or DVD. In Windows 7 or 8, you can right-click the ISO and choose Burn disc image; for earlier versions of Windows, use a third-party tool such as the free CDBurnerXP. Make sure any option to make the disc bootable is ticked.

If you don’t have a CD or DVD drive, you can use a USB stick. Download the free Rufus utility to easily create a bootable USB drive from the downloaded ISO file. Be sure that “Create a bootable disk using:” is ticked and its dropdown is set to “ISO Image”, then click the small icon to the right and navigate to the DBAN ISO file.

Erasing a disk with DBAN

Once you’ve created your DBAN media, boot your PC from it and at the first menu simply press Enter to begin the process. At the Disks And Partitions screen, press Space on each of the drives you intend to wipe. You’re free to change the random-number generator (press P), the wipe method (W), and the verification options (V), but it’s fine to stick with the defaults – the standard settings are more than robust enough.

As for the number of rounds (R), it’s often claimed that more is better, but in fact overwriting a drive only once will almost certainly make it unrecoverable, even by forensic experts with specialist hardware. Each DBAN round writes three passes of random data, so we’d leave this at the default of one.

When you’re absolutely certain you’re ready to start wiping your drive, press F10. The screen will fill with text, before being replaced by a running progress report as each drive is run through three passes before being verified as empty. DBAN took close to eight hours to fully wipe our 500GB laptop hard disk, so put the kettle on and make other plans for the day.

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