If you decide to sell or give your PC to someone else, it’s advisable to delete all of your personal files beforehand — for obvious reasons. The problem is that actually deleting anything on a computer isn’t as easy as it sounds.
Even if you empty the Recycle Bin, it’s usually easy to recover deleted files using freely available software and that raises some important security concerns.
What happens when Windows deletes a file?
The problem is that when you delete a file in Windows, the file isn’t actually “deleted” as such. Instead, Windows simply marks the space it occupies on the hard drive as “empty” so that it can be used to store something else and the data that makes up the file simply stays on the drive until then.
So what can you do when you really do want to delete a file forever so that it can’t be restored? The answer lies in what happens to a deleted file once part of it has been overwritten with something else.
As soon as a new file uses some of the disk space an old file used to occupy, the chances of undeleting that old file diminish. Some file recovery software can reconstruct files even if some of the data has been overwritten (recovering something is better than nothing, in some cases) but this becomes impossible beyond a certain point. Check out our guide to recovering deleted files on Windows 10 here.
How secure delete works
That’s just how secure delete software works. Rather than simply mark a file as “deleted” but leave its data in place, secure delete software actually overwrites that space with other data. In most cases, simply overwriting it with ones or zeros (which is how data is actually represented on a hard drive) is enough to render it unrecoverable, but it is still possible to analyse a hard drive using a microscope to discover the “ghost image” of overwritten data — not that anyone buying a second-hand PC via Ebay would bother.
That said, the size and sophisticated storage technology of modern multi-terabyte hard drives make this already difficult technique effectively impossible, but writing over old files several times with random data destroys it irrevocably.
Smash and grab
Of course the most paranoid PC users will still claim that the only way to ensure the contents of a hard drive can’t be recovered is to remove the drive, disassemble it and physically destroy the magnetic platters that store the data. That’s probably taking things a bit far for most purposes, but it’s still far from foolproof if someone really, really wants to get that data back. It may be prohibitively expensive, but it can be done.
Kroll Ontrack is a data recovery company that specialises in such difficult cases. It has successfully downloaded photos from digital cameras that were all but destroyed in fires, for example, and retrieved 99% of the data from a hard drive recovered from the remains of the 2003 Columbia Space Shuttle disaster.
So if you want to securely delete files stored on your hard drive, the simple option is to use a free secure delete utility. There are a few available, but we’ve chosen Eraser, which supports Windows XP or upwards, including Windows 10. Remember, you download third-party software at your own risk - ensure you read all the installation steps carefully and if you aren't happy, don't use the software.
Securely delete your files
Step 1: Download the free Eraser tool
Go to http://eraser.heidi.ie and click Download at the top. Look for the download link under Build Name partway down the page. Click the link called Eraser 184.108.40.20682.
Step 2: Start the download from the SourceForge site
The download link will take you to a site called SourceForge, which is a hosting site for open-source software. The download will begin automatically after a few seconds.
Step 3: Install Eraser
Double-click the downloaded .exe file to begin the installation and select the Typical installation option. There’s nothing to catch you out with the installation, otherwise.
Step 4: Right-click a file to delete it securely
To securely delete any file, right-click it and select Eraser - Erase from the menu that appears. Click the Yes button on the dialog box to continue — and bear in mind that you won’t be able to recover the file afterwards.
Step 5: Change the secure delete options
If you click the Options button on the dialog box, rather than Yes, you can double-click the Erasure Method and change it from the default overwrite option to one that involves overwriting a file repeatedly. This really isn’t necessary, though, and can make the delete process much slower.