Securely Wiping a Hard Disk
So you've bought a new computer and decided to be generous and donate your old one to a charity, or perhaps you've decided you need some more space and have upgraded your hard drive and are looking to sell the old one to recoup some cash. You've remembered to format the drive, so you're safe from someone reading your files, right?
Well, no. You see, deleting files and formatting drives still leaves your files perfectly readable using freely available software. A study by the Information Commissioner's Office found everything from bank statements to scanned passports on hard drives bought online and at computer fairs. Criminals use old hard drives as a source of personal information to commit identity theft.
So how do you securely delete your files? There are a number of free, simple options. If you want to do the whole disk, my personal preference is Darik's Boot and Nuke (DBAN), just download and burn onto a CD, boot from the CD and away you go (obviously be very careful you don't remove anything you want to keep).
If you have a Mac, you can use the built-in Disk Utility (under /Applications/Utilities). And if you want to do individual files, give Eraser (for Windows) a try. On a Mac, delete the file and then select Secure Empty Trash to permanently remove.
What about if your drive has failed? Well people can still get the files off by extracting the platters, though it's much harder work. You can use a hammer and some heavy blows or a drill to damage it beyond repair.
It's not difficult to ensure your data stays safe, just remember to take the right precautions.