The fact that accidentally deleted data isn’t necessarily gone forever might come as something of a relief but there are worrying consequences too. If you can’t properly delete the data on your hard disk prior to selling or disposing of your PC, or merely allowing someone else to use it, the consequences could be disastrous.

When you need to destroy data permanently, there are some rather extreme methods to render the entire disk unreadable (we'll get to these later) but these do rather depreciate the value of your PC. Here, we’ll show you how to securely delete your data while keeping your disk intact.

The method is simple enough and there are several software packages that provide secure deletion facilities. In addition to removing reference to the file in the disk’s directory, the software overwrites the areas of the disk occupied by the file with random data. See also: How to delete data using Eraser 6

The exact process differs depending on the file system and because flash drives tend to use a different file system from the hard disk, you should look for an application that supports both FAT variants and NTFS.

Delete key

Interestingly, it has been suggested that overwriting a file with random data isn’t enough. The argument is that when data is written to a disk, the resultant magnetic flux has a tiny contribution from the data previously occupying that part of the disk. It would follow, therefore, that if you extract the analog signal directly from the disk’s read/write head, and subtract the signal corresponding to the current data, you’re left with that tiny contribution from the previous data.

For this reason, some secure deletion software overwrites not just the once but many times. We discussed this with an expert in hard disk technology at Western Digital who told us that he didn’t know of anyone ever having achieved it. And while not suggesting that it was impossible, he added that it would be so expensive and time consuming that he couldn’t imagine it being attempted except by government agencies when national security was at risk.

So although there’s no reason to avoid software that claims to overwrite files 40 times, this shouldn’t be a criterion in selecting a suitable product.

Optical media

Optical discs, such as the variants of CD, DVD and Blu-ray, are quite different from magnetic or flash drives and most secure deletion products won’t touch them. For rewritable dics this really isn’t an issue since you’re not going to be disposing of them along with your PC as you would with the hard drive. If you do decide to give an optical disk to someone it’s normally specifically because you want them to have your data.

However, write-once disks such as CD-R, DVD-R and DVD+R are quite different because you’ll throw them away when you’ve finished with them. In this case, the way to prevent your data falling into the wrong hands is to physically destroy the disk. You can buy paper shredders which will also shred optical disks and if you’re paranoid you might choose that solution.

However in our experience, low-cost home shredders really aren’t tough enough for this sort of treatment and shredding CDs on a regular basis will probably contribute to premature failure.

DVD cut in half

Nobody is going to spend countless thousands on leading edge data recovery in the off-chance that a CD-R they find in your bin contains valuable data so just ensuring that it’s in two pieces will be enough. Cutting it with scissors (not your best pair, though) is one option or you could just snap it in two, having first wrapped it in a cloth to protect yourself from flying shards of plastic.

Next page: we take an angle grinder to a hard disk