A hard drive works by flipping magnetic bits around on a metal plate to represent the 1s and 0s that represent binary data. When you delete a file, you are changing it so the operating system doesn't see it so it can overwrite it with new data. The data is still there, and it can still be retrieved using recovery tools. Even physical damage may not stop data recovery: If someone really wants to get your data, they can buy an identical hard drive and use it to repair your current one, or have the disks examined under microscope by a specialist.
There are two guaranteed ways to completely destroy your data:
Overwriting: The easiest way to wipe an entire hard drive is by using Darik's Boot and Nuke (DBAN), a free program that will completely rewrite your hard drive. To use it, you'll need to burn it to a CD and use it as a boot disk. This way nothing from the hard drive is loaded, so it can be completely deleted. When run, it will rewrite the entire hard drive, eliminating any trace of data.
If you want to leave the operating system intact, you will need to use a program to delete and write over individual files. There are several free programs available to do this including File Destroyer and File Deleter for Windows and Permanent Eraser for OSX. Linux users can use the built-in "shred" command.
Giant magnet: Yes, really. If you work in a large office that handles a lot of sensitive data, they may have a hard drive degausser. This large high frequency magnet changes the states of all those magnetic spots on the hard drive almost instantly. This probably isn't an option for home users since one of these magnets costs as much as a car.
Posted 4272 day ago