August 11, 2011
There's been an ongoing controversy about what you have to do to make sure no one can recover data off of your hard drive after you've erased it. You've probably heard that just deleting files doesn't really erase them from the disk. This is true. When you delete a file, the operating system simply marks the spaces used by that file as being empty; the data is still there and can be recovered by any simple disk editor utility. You've probably also heard that in order to truly get rid of the data in those spaces, you have to overwrite it many times. People throw around numbers like 10 to 35 overwrites before the data is securely wiped. This is [i:14vlny1g]not[/i:14vlny1g] true. You only have to overwrite the old data [i:14vlny1g]once[/i:14vlny1g] to make it impossible to recover. That's good news (although the people who write secure erase software probably don't want to hear it).
In my latest posting to [url=http://itknowledgeexchange.techtarget.com/security-corner:14vlny1g]Security Corner[/url:14vlny1g], "[url=http://itknowledgeexchange.techtarget.com/security-corner/the-great-drive-wiping-controversy-settled-at-last/:14vlny1g]The Great Drive Wiping Controversy Settled at Last[/url:14vlny1g]," I give the details on research that proves beyond a doubt that a one-pass overwrite of deleted data makes the original data unrecoverable, even with an electron microscope.
There are plenty of secure erase programs and "file shredders" out there, many of them free. So, don't buy one. My favorites are [url=http://technet.microsoft.com/en-us/sysinternals/bb897443.aspx:14vlny1g]SDelete[/url:14vlny1g] from Mark Russinovich, and [url=http://www.dban.org/:14vlny1g]DBAN[/url:14vlny1g] (Darik's Bute and Nuke) which is Open Source. Use SDelete from within Windows to overwrite all of the free space (where your deleted files are sitting) on your hard drive. Use DBAN when you want to completely wipe an entire hard drive. Both programs let you specify how many passes to make: Now you know it only takes one pass.
If you own a Mac the [url=http://www.apple.com/downloads/macosx/system_disk_utilities/permanenteraser.html:14vlny1g]Permanent Eraser[/url:14vlny1g] utility builty into OS 10.4 and later has a "Zero Out Data" option which writes zeros over the deleted. The utility can also overwrite the data 7 times or you can use the Gutmann method (see my article) to overwrite it 35 times. No sense in spending 7 times or 35 times as much time on it--just zero out the data and you're good to go.
As always, questions, comments, and flames are welcome!
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