Why Erasing Files Is Not Enough
Sure, you could erase the contents of the drive, but keep this in mind: the act of erasing a file does not remove it from a storage device.
When you erase/delete a file from your computer, it's not really gone until the areas of the disk it used are overwritten by new information. If you use the normal Windows delete function, the "deleted" file is sent to the Recycle Bin until the space it uses is required by other files. If you use Shift-Delete to bypass the Recycle Bin, the space occupied by the file is marked as available for other files. However, the file could be recovered days or even weeks later with third-party data recovery software. As long as the operating system does not reuse the space occupied by a file with another file, the "deleted" file can be recovered.
With SSDs, the erased file situation is even more complex. SSDs store data in blocks rather than in sectors as with magnetic storage. Overwriting a block was previously used involves copying the contents of the block to cache, wiping the block's contents, delete the block to be overwritten from cache, writing the new data to cache, and rewriting the block with the new data. As an SSD is used with files that are deleted or changed frequently, the performance can drop unless the drive (and operating system) support a technology called TRIM that wipes out deleted data blocks as soon as the file using the blocks is deleted. TRIM is supported by Windows 7 and by some late model SSDs, but not by older Windows versions. So, disk wiping can be both a security feature and a performance improvement strategy.
Data Wiping Versus File Erasure
While erasing files simply marks file space as available for reuse, data wiping overwrites all data space on a storage device, replacing useful data with garbage data. Depending upon the method used, the overwrite data could be zeros (also known as "zero-fill") or could be various random patterns.
Products that can be used for wiping hard disks might not be suitable for wiping other types of drives. In this article, we will look at methods for securely wiping hard disks, USB flash memory devices, flash memory cards, and SSDs.
Zero-Fill a Hard Disk
Time Needed: several hours (varies with size and speed of drive)
Software: Hard disk utility software from your drive vendor
Media: blank CD or floppy disk
Although writing zeroes across the entire hard disk surface (aka "zero-filling") is not sufficient to meet government data sanitation (disk wiping) standards such as DoD 5220.22-M or the more comprehensive Standards and Technologies (NIST) Special Publication 800-88, overwriting the entire hard disk prevents most types of data recovery from being successful.
Here's where to get zero-fill software from hard disk vendors:
Hitachi
Drive Fitness Test (see website for specific models supported)
http://www.hitachigst.com/hdd/support/download.htm#DFT
Select the Erase Drive feature to zero-fill your hard disk
Samsung
HUtil (see website for specific models supported)
http://www.samsung.com/global/business/hdd/support/utilities/Support_HUTIL.html
Use Tool, Erase HDD to zero-fill your hard disk
Seagate (including Maxtor)
SeaTools for DOS (see website for specific models supported)
http://www.seagate.com/www/en-us/support/downloads/seatools
Use Full Erase to zero-fill your hard disk
Western Digital
Data Lifeguard Diagnostics (select drive model for specific version recommended)
http://support.wdc.com/product/download.asp?lang=en
Use Write Zeros to drive to zero-fill your hard disk
1. Determine the brand and model of hard disk you want to overwrite.
2. Download a CD ISO image or a floppy disk image (depending upon your equipment) and use the image to create bootable media. The floppy disk image is self-contained: run it, insert a blank floppy disk when prompted, and the image is created on the disk. You will need to use a CD burning program that works with ISO images to convert the ISO image into a bootable CD.
3. Restart your computer with the bootable media you created in Step 2.
4. Select the hard disk to zero-fill when prompted.
5. Choose the option to zero-fill the hard disk.
6. When the program is finished, follow the on-screen instructions to shut down or restart your computer.
7. Remove the wiped hard disk; you can now reuse or recycle the hard disk.
Secure Wiping a Hard Disk
Secure wiping goes beyond zero-fill operations, and provides an extra level of security. Most secure wiping programs are designed to meet DoD 5220 standards, which require three passes of overwriting with a special numeric pattern and verification. More information about this and other secure standards are available from the DataErasure website.
(Note that the 2007 revision of the Defense Security Service, Updated DSS Clearing and Sanitization Matrix (June 28, 2007) (PDF) now recommends degaussing or drive destruction for maximum protection.
Stanford University's Disk and Data Sanitization Policy and Guidelines, a must-read for understanding data wiping issues, recommends Darik's Boot and Nuke (DBAN) for secure hard disk wiping.
Secure Wiping a Hard Disk with DBAN
Time Needed: several hours (varies with size and speed of drive)
Software: Darik's Boot and Nuke (DBAN); available from http://www.dban.org/
Media: blank CD (all versions) or floppy disk (version 1.0.7 and older versions)
1. Download the DBAN boot image ZIP file (we used version 1.0.7 and beta version 2.0 for this article); we downloaded the ISO image for CD burning, but a floppy disk builder is also available
2. Extract the contents of the compressed file.
3. Burn the ISO image file extracted in Step 2 to CD; see our article on how to do this, or use the built-in ISO CD image burning support in Windows 7. If you downloaded the floppy image builder, run the program to create a bootable floppy disk.
4. Restart the computer using the CD or floppy disk created in Step 3.
5. Press Enter to run DBAN in interactive mode.
6. Use up and down arrow keys to highlight the drive to wipe.
7. Press the space bar to select the drive.
8. Press M to select the wiping method.
9. Press F10 to begin the wipe process.
10. At the end of the process, shut down the system. You can reuse or recycle the wiped hard disk.
Note: if DBAN is unable to recognize your SATA hard disks, configure your system BIOS to use IDE mode rather than AHCI mode.
Wiping Flash Memory Cards and USB Drives
Programs such as DBAN or vendor-supplied hard disk utilities are limited in the devices they support: they are designed to work with internal ATA/IDE or SATA hard disks only. Programs that work with flash memory cards and USB flash drives often support hard disks as well, enabling you to use a single program for all disk wiping processes. Roadkil's DataWipe can be used with any hard disk, floppy disk, or flash drive that has a drive letter.
Wiping Flash Memory Cards with Roadkil's DiskWipe
Time Needed: Varies; from a few minutes to several hours, depending upon size and speed of drive and computer
Software: Roadkil's DiskWipe, available from http://www.roadkil.net/
Media: Can be run from Windows desktop
1. Download Roadkil's DiskWipe.
2. Extract the contents of the compressed file.
3. Open DiskWipe. If you are running Windows Vista or Windows 7, right-click the program icon and select Run as Administrator.
4. Select the drive to wipe.
5. Select the type of wipe to perform; DiskWipe can zero-fill the disk or write random data.
6. Enter the number of passes.
7. Click Erase to start the process.
8. At the end of the process, close the program. You can reuse the wiped disk.
Wiping SSDs
To solve write performance problems on drives that don't support TRIM (check with your drive vendor for firmware upgrades) is to use wiper.exe (included with some SSDs) or to run the Secure Erase feature supported in most recent ATA/IDE and SATA drives. The Secure Erase feature can be activated on many systems by running Secure Erase 4.0 (HDDerase.exe), available from http://cmrr.ucsd.edu/people/Hughes/SecureErase.shtml. Version 4.0 works with most recent ATA/IDE and SATA hard disks and SSDs, but if you use an Intel X-25M, X-25E, or X-18M SSD, follow this link to download Secure Erase 3.3 http://www.iishacks.com/index.php/2009/06/30/how-to-secure-erase-reset-an-intel-solid-state-drive-ssd/. Note that it is no longer being developed, and we were unable to use it on a system running an AMD 690 chipset.
Wiping Drives and Free Space with SDelete
SDelete is a free program from Microsoft's TechNet Sysinternals collection. It runs from the command line, and can be used to wipe drives, wipe files, or wipe free space.
Time Needed: Varies; from a few minutes to several hours, depending upon size and speed of drive and computer
Software: TechNet Sysinternal's SDelete, available from http://technet.microsoft.com
Media: Can be run from Windows desktop
1. Download SDelete.
2. Extract the contents of the compressed file.
3. Copy sdelete.exe to c:\windows\system32\ (this will enable you to run it from any location)
4. Open a command prompt session with Administrator rights.
5. To wipe all files on drive X: and its subdirectories and to wipe free space, enter Sdelete -p 2 –s -z X:\*.* (to see all command-line switches, enter Sdelete with no options)
6. Wait; the program displays status messages as it runs. When the program is finished, you can reuse or dispose of the drive.
Evaluating the Effectiveness of Disk Wiping Programs
We used demo versions of two popular data recovery programs to evaluate some of the disk wiping programs discussed in this article. To determine whether a typical data recovery program could recover files on a SD card wipe with Roadkil's DiskWipe, we first of all formatted the card using a card reader. Ontrack's EasyRecovery Data Recovery (available from http://www.ontrack.com) had no difficulty finding folders and files to retrieve.
However, when we used DiskWipe to wipe the drive using a one-pass blank disk (zero fill) operation, EasyRecovery DataRecovery was unable to find the file system, let alone any files or folders.
After reformatting the card, taking a few photos, and deleting the photos, EasyRecovery Data Recovery was able to find the new photos, but the contents of the card before running WipeDisk were unrecoverable.
To evaluate SDelete, we used SDelete to wipe all of the files on a hard disk, but omitted the –z switch; when –z is not used, SDelete deletes files and renames them, but does not clear free space. To determine what might be visible, we used a demo version of Disk Doctors NTFS Data Recovery software, available from http://www.diskdoctors.net.
Disk Doctors were able to locate the deleted folder and Outlook Express message folders, but SDelete had renamed them from their original names and DBX extensions (Outlook Express message folders). If you use SDelete, it's very important that you take time to use the –z switch to clear free space on the disk (once a file is deleted, the space it occupies is free space).
We also used Disk Doctors to evaluate the effectiveness of a freeware program called Eraser, which can delete and overwrite files and folders from the right-click menu. We created a documents folder with a subfolder called Figures and used Eraser to overwrite the folder and subfolder using its default settings.
Disk Doctors was able to locate the folders, but the contents are files with garbage names and are zero bytes in size – except for leftover word processing temporary files (files that begin with $). These filenames were not changed, which could enable a snooper to figure out the names of the files in the folder – although the files themselves were destroyed. By using more overwrites or different methods available with Eraser, a more thorough wiping may be possible.
Conclusion
We've highlighted a variety of free ways to protect data on castoff drives from being retrieved. As you can see, your best bet is to overwrite data directly, but you also might want to consider using a program such as SDelete to scramble filenames first and then use a disk wiper such as Eraser or WipeDisk to finish the job.
Use demo versions of data recovery programs such as Ontrack Easy Recovery Data Recovery, Disk Doctors Data Recovery (various editions for NTFS, FAT, and flash media), and others to evaluate the effectiveness of your data wiping procedures. Remember, the full versions of these and other data recovery programs can save your data if you accidentally format or partition a disk because, until the data is overwritten, it's still there.
Note: We originally ran this piece on March 10th, but thought it was so appropriate for memory week that we've brought it back. Memory [Forever] is our week-long consideration of what it really means when our memories, encoded in bits, flow in a million directions, and might truly live forever.
