If you want to completely erase a hard drive, it's not as easy as deleting everything on it. To truly erase hard drive data forever, you'll have to take some extra steps.
When you format a hard drive you don't actually erase the hard drive of data, you only erase the location information for the data, making it "lost" to the operating system. Since the operating system can't see the data, the drive looks empty when you look at its contents.
However, all the data is still there and, unless you truly erase the hard drive, can be recovered using special software or hardware.
The most responsible thing you can do before recycling a hard drive, or even disposing of one, is to completely erase the hard drive. If you don't erase the hard drive, you risk exposing sensitive personal data that you previously deleted - data like social security numbers, account numbers, passwords, etc.
According to most governments and standards organizations, there are only three effective methods of erasing a hard drive:
By far, the easiest way to completely erase a hard drive is to use free data destruction software, sometimes called hard drive eraser software or disk wipe software.
Regardless of what you call it, a data destruction program is a piece of software designed to overwrite a hard drive so many times, and in a certain way, as to make the ability to extract information from the drive nearly impossible.
Some more stringent hard drive erasing standards forbid using data destruction software, probably because of the possibility of user error and the variety of software and methods that exist. However, as long as your drive doesn't contain national security information, you should feel very comfortable using any one of these programs to erase a hard drive.
Important: You must erase a hard drive using this method if you, or someone else, ever plans on using the drive again. The next two ways to erase a hard drive will make the drive unusable. For example, you should erase a hard drive this way if you're selling or giving the drive away.
2. Use a Degausser to Erase the Hard Drive
Another way to permanently erase a hard drive is to use a degausser to disrupt the magnetic domains on the drive - the very way that a hard drive stores data.
Some NSA approved automatic degaussers can erase dozens of hard drives in an hour and cost tens of thousands of dollars US. NSA approved degaussing wands, used to manually degauss a hard drive, can be purchased for around $500 USD.
Important: Degaussing a modern hard drive will also erase the drive's firmware, rendering the drive completely useless. If you want to erase a hard drive, but also want it to work properly after being erased, you must erase the drive using data destruction software (option 1, above) instead.
Note: For the average computer owner or organization, degaussing probably isn't a cost effective way to completely erase a hard drive. In most cases, physically destroying the drive (below) is the best solution if the drive isn't needed any more.
3. Physically Destroy the Hard Drive
Physically destroying a hard drive is the only way to absolutely and forever ensure that the data on it is no longer available. Just as there is no way to extract the written information from a burned piece of paper, there is no way to read the data from a hard drive that is no longer a hard drive.
According to the National Institute of Standards and Technology Special Publication 800-88 [PDF], "Destruction of media is the ultimate form of sanitization." Most of the standards that exist to erase a hard drive mention several ways to physically destroy one including disintegration, grinding, pulverization, incineration, melting, and shredding.
You can destroy a hard drive yourself by nailing or drilling through it several times, making sure the hard drive platter is being penetrated each time. In fact, any method of destroying the hard drive platter is sufficient including sanding the platter after being removed.
Warning: Wear safety goggles and take great caution destroying a hard drive yourself. NEVER burn a hard drive, put a hard drive in a microwave, or pour acid on a hard drive.
If you'd rather not destroy your hard drive yourself, several companies offer the service for a fee. A few services will even fire a round of bullets through your hard drive and send you the video!