My eldest daughter was distraught recently after she deleted the digital photos of her holiday stored on a flash memory card. But were they really gone? Like my daughter, most people wrongly believe that when they delete a file or a folder on a hard drive or other storage device, it is gone for ever.

In reality, it is quite difficult to erase data permanently once it has been stored on a digital device. That is good news if you need to retrieve it but bad news if you really did intend to erase it – if, for example, you are giving an old PC away and its subsequent recovery by someone else compromises your privacy or worse.