When you erase a file on your computer, the actual data in the file is not overwritten. The space utilized by that file is simply marked as “free” for use by other data.
Once other data is written in that space, the original data becomes unrecoverable — by most people.
Those little bits of magnetic media that store 0’s and 1’s are a bit more complex, however. With sensitive and sophisticated equipment, it is theoretically possible to recover data even after it has been written over.
The method to securely erase data is to write over the same physical spot on the hard disk multiple times with different patterns, effectively obliterating the magnetic signatures of the data which was once there.
Standards for Securely Erasing Data
The Defense Security Service (DSS) of the Department of Defense (DoD) has published standards for the secure deletion of magnetic media in the National Industrial Security Program Operating Manual (NISPOM): Operating Manual Supplement, as modified by the Department of Defense Overprint.
Peter Gutmann of the University of Auckland has written a better paper on the topic, Secure Deletion of Data from Magnetic and Solid-State Memory.
Software to Securely Erase Data
Many programs are available that are designed to securely erase data from magnetic media:
- SDelete by Mark Russinovich
- Eraser by Sami Tolvanen
- NecroFile by The Nth System
- Shred File 2002 by the Nth System
- File Shredder