For the general user, the delete or format command appears to be the logical method of removing unwanted data files. These methods, however, are like sweeping something under the carpet: you may not be able to see it, but it's still there. All that deletion has done is remove the pointer to the files, with the data itself residing in unallocated space on the hard drive. This means that data recovery is possible using various software tools.

When sensitive information is stored on the hard drive of a machine that is to be surplussed or transferred to another individual or department, it is therefore imperative that extra measures be taken to wipe clean the hard drive before the computer leaves your area of responsibility. This document describes some common methods and software to assist you with the sanitization process. It also includes links to articles that provide detailed technical descriptions of what occurs during this process.

The NIST Special Publication 800-88, Guidelines for Media Sanitization, provides an overview of sanitization techniques and requirements. According to the publication, the purpose of sanitization is "to render access to target data (the data subject to the sanitization technique) on the media infeasible for a given level of recovery effort", and describes the three categories of action to be taken to sanitize media as:

• Clear applies logical techniques to sanitize data in all user-addressable storage locations for protection against simple non-invasive data recovery techniques; typically applied through the standard Read and Write commands to the storage device, such as by rewriting with a new value or using a menu option to reset the device to the factory state (where rewriting is not supported).
• Purge applies physical or logical techniques that render Target Data recovery infeasible using state of the art laboratory techniques.
• Destroy renders Target Data recovery infeasible using state of the art laboratory techniques and results in the subsequent inability to use the media for storage of data.

See Section 5 of the document for a more detailed summary of sanitization techniques, and Appendix A for sanitization requirements for specific media/device types.

The three most common techniques for properly sanitizing hard drives are:

• Physically destroying the drive, rendering it unusable. This is a good alternative for defective hard drives or those that would be too costly to repair. For added security, the disk should be overwritten or degaussed prior to destruction.
  N.B. Hard drives, cell phones, and PDAs that are no longer needed and contain data covered under the Brown Restricted Information Policy can be brought to the offices of ISG for crushing and disposal. Please contact ISG to schedule an appointment.
• Degaussing the drive to randomize the magnetic domains – most likely rendering the drive unusable in the process. Degaussing, or demagnetizing, applies a reverse magnetizing field to data stored on magnetic media, erasing the contents by returning the magnetic flux to a zero state.
• Overwriting the drive's data so that it cannot be recovered. Overwriting replaces previously stored data on a drive or disk with a predetermined pattern of meaningless information, rendering the data unrecoverable.

Note that when removing sensitive information, don't forget storage devices such as thumb drives, back-up external hard drives and CDs. Also, be sure to erase any stored names and numbers from phones and fax machines.

EXTRA: OIT has a hard drive crusher used for crushing no-longer needed drives containing data covered under the Brown Restricted Information Policy. Contact the IT Service Center for more details and to arrange an appointment.

The following chart is a collection of disk wiping software recommended by departmental computing coordinators (DCCs) or listed on a variety of other University and security sites. The inclusion of any title does not indicate an endorsement by Brown University or the Office of Information Technology, and has only been provided as an aide in making a decision that best matches your specific needs. See also the Wikipedia list of data-erasing software.

4.1 Windows

Each of the software products listed above comes with specific instructions, some with an easy-to-use wizard interface. KillDisk (recommended by some DCCs) is the software of choice at Northern Illinois University. Their support for this product includes detailed instructions on its use. Dell offers an overview document How Do I Erase Data from My Hard Drive?.

4.2 Macintosh

In addition to the software offered above, Mac computer hard drives can be cleared by zeroing their data. Note that zeroing data (aka "low level" format) may take a long time and depends on the hard disk size. It is recommended to use the "8-way random" feature in conjunction with the "zero all data" option.  See the section "Securely erase a disk" in the article Disk Utility 12.x: Erase a Disk, CD or DVD for details.

4.3 Solaris / Linux / Unix

• Secure Erase in Unix / Linux / Solaris (2006)
• Solaris: Erasing Disks Securely (2011) | Explicit ZFS Data Scrubbing (2011)
• Linux: Secure Deletion of Data in Linux (2012) | How to Delete Files Securely (2007)
• Unix: DOD compliant disk / file wiping (2007) | Secure File Deletions (SANS GIAC paper, 2001)

Compendium of disk wiping software:

• Darik's Boot and Nuke (DBAN): sourceforge.net/projects/dban/
• Disk Utility: support.apple.com/kb/PH5849
• Disk Wipe: www.diskwipe.org
• East-Tec DisposeSecure: east-tec.com/disposesecure/
• Eraser: sourceforge.net/projects/eraser/
• KillDisk (Active@KillDisk): killdisk.com/
• Linux: linux.com/learn/tutorials/442455-wiping-your-disk-drive-clean
• Paragon Disk Wiper: http://www.paragon-software.com/home/dw-professional/
• sDelete: technet.microsoft.com/en-us/sysinternals/bb897443.aspx
• secure rm: sourceforge.net/projects/srm/
• ShredIt: mireth.com/shredit.html
• Wipe: sourceforge.net/projects/wipe/
• WipeDrive: whitecanyon.com/ConsumerWipedrive | whitecanyon.com/EnterpriseWipedrive

Further Reading:

• ST18-005: Proper Disposal of Electronic Devices, US-CERT (October 2018)
• Guidelines for Information Media Sanitization EDUCAUSE, Information Security Guide Toolkit (September 2015)
• Special Publication 800-88: Guidelines for Media Sanitization, by the National Institute of Standards and Technology, NIST (revised December 2014)
• Securely Disposing of Computers and Other Storage Devices by Rob Lee, SANS' OUCH! newsletter (January 2011)
• Sanitizing Media (The Linux Method) by Hal Pomeranz, SANS Computer Forensics blog (June 2010)
• Secure File Deletion, Fact or Fiction? by John R. Mallery, SANS Institute (June 2006)
• Remembrance of Data Passed: A Study of Disk Sanitization  by Simson L. Garfinkel and Abhi Shelat, MIT, IEEE Computer Society, Security & Privacy, vol. 1, no. 1 (2003)
• Deleting Sensitive Information:Why Hitting Delete Isn't Enough by Hans Zetterstrom (2002)
• What You Don't See On Your Hard Drive by Brian Kuepper, SANS Institute (April 2002)
• Securely Deleting Files by John Kinney, SANS Institute (2002)

Related sites at other universities:

• Carnegie Mellon: Data Sanitization and Disposal Tools
• Stanford University: Disk and Data Sanitization Policy and Guidelines
• Syracuse University: Data Removal from Computers and Portable Electronic Devices
• University of Minnesota OIT Security: Media Sanitization