According to the New York Times, a basic privacy measure that is often overlooked is the proper destruction of data on hard drives. An ongoing study by British Telecom says that most people don't realize that deleting a file doesn't actually remove the data from a computer.

In fact, the BT research found that only 33 percent of second hand hard drives had been completely wiped clean. To ensure your drive doesn't contain any personal data before you give it away or sell it, you need to reformat the hard drive or use digital shredding software if you want to completely eliminate all traces of data. In this post, we'll show you how.

When it comes to data stored on your computer, deleting files doesn't actually remove the data. File information is kept in a directory so that the operating system can find it. When you delete a file, all you are doing is removing it from the directory and flagging that part of the drive as being available for new data. Until that region is overwritten, the old data can be retrieved, in fact that's how you can recover lost data. It's also the way most file recovery programs work - they look for data on your hard drive that shouldn't be there according to the directory and restore it.

The only way to completely remove the data is to overwrite the contents of the hard drive. You can do this by formatting the drive, or using data wiping software that fills your hard drive with random data.

Two Main Methods of Overwriting Data

The Gutmann Method

Based on Peter Gutmann's paper "Secure Deletion of Data from Magnetic and Solid-State Memory", this method provides the best security. Data is overwritten 35 times with carefully selected patterns, which makes it unrecoverable. Unfortunately, it also makes it time consuming.

US DoD 5220-22.M

Based on the United States Department of Defense recommendation 5220-22.M, this method overwrites the data seven times. While less secure than the Gutmann method, it is faster.

Mac Users: Data Wiping Built In

Macs come with data wiping tools built into their systems. To securely delete a folder or file, all you need to do is move it to the trash can and from the Finder menu select 'Secure Empty Trash'.

Alternatively, the Disk Utility program 'Erase Free Space' scans your hard drive for unused space and securely deletes it to military (7 passses) or Guttman (35 passes) standards. You can find it in the Utilities folder.

Windows: Data Wiping by Selected File/Folder

Eraser is a free, open source program that works with Windows 95, 98, ME, NT, 2000, XP, Vista, Windows 2003 Server and even DOS. It uses carefully selected patterns to overwrite your hard drive several times and lets you select single files, entire folders or the entire drive to be wiped clean.

How to Use Eraser

Download and install the application. Once running, you'll be presented with a simple box that allows you to either schedule an erase, or do it there and then (on demand).

There are three ways to enter data into the list if you are doing it on demand:

1. Select files and folders in Explorer and drag and drop them to the list
2. Copy them to the clipboard and then paste them to the list
3. Use the New Task command in the File menu.

Note: If you use the New Task command, a window will appear allowing you to select unused space on a drive, a folder or a file to be erased.

After you have added the data you wish to erase to the list, you need to select the method of removal.

  1. The Gutmann Method (Default)
  2. The US DoD 5220-22.M Method
  3. The Pseudorandom Data Method

Using the pseudorandom data method, all passes will be random data, which is highly incompressible. This is the only method that should be used when erasing unused space or data on a compressed drive. The number of passes is user selectable from one to 65535.

Once you've selected the files/folders/drives that you want deleted, and the method of deletion, you can run Eraser. It will ask you for confirmation before it starts erasing so make sure you are certain you want to continue as this is your last chance to prevent data from being accidentally erased.

Data Wiping of Complete Drive

Another free open source program that runs on Windows virtually any machine, Darik's Boot and Nuke (DBAN) wipes the entire contents of every hard drive it finds on your computer when you run it, so be very careful.

From the DBAN about page:

Darik's Boot and Nuke ("DBAN") is a self-contained boot disk that securely wipes the hard disks of most computers. DBAN will automatically and completely delete the contents of any hard disk that it can detect, which makes it an appropriate utility for bulk or emergency data destruction.

DBAN is simple enough to use. Once you've downloaded the program, write it to a CD, DVD, or thumb drive, boot from that and enter 'autonuke' at the prompt.

The New York Times has an in depth article about DBAN that includes an interview with the author, Darik Horn if you are interested in further reading.

The Ultimate Permanent Erase

While these methods of data wiping can be useful for most of us; if you're uncertain about using them, there is one other option.

  1. Remove the hard disk from the computer
  2. Unscrew the casing, exposing the disks
  3. Smash them to smithereens

Your data now remains private. :)

Update: As pointed out by Stu in the comments, simply formatting the contents of the hard drive is insufficient if you want to delete the data; the drive will also need to be overwritten to ensure data deletion. Additionally, a 'quick-format' will not suffice, it must be a full format.