Helping people with computers... one answer at a time.

Secure Delete is a way to make sure that when you delete a file it cannot be recovered. Understanding Secure Delete is important to protect sensitive data properly.

Although there are many software utilities that claim to be able to delete data files from hard drives securely and thoroughly, can't you accomplish the same thing simply by overwriting sensitive files with large, non-sensitive ones?

To be honest, it depends on your level of paranoia. I suppose that also depend on the level of sensitivity of your data.

But you are correct in the implication that a plain old "delete" isn't nearly enough.

Let's look at that, and how far you might need to go.

As you may already know, deleting a file in Windows doesn't actually delete the data. In fact, it doesn't really even delete the file - in Windows Explorer if you delete a file it just gets moved to the recycle bin. The file's not really deleted until the recycle bin gets too full, or until you empty it yourself.

Naturally it's trivial to go digging around in the recycle bin to see what's been "deleted", and recover it intact.

"Because of the way magnetic material on hard disks works, it might be possible to actually recover data that has been overwritten."

Even a "permanent" delete after, or bypassing, the recycle bin doesn't really delete the data. In a sense, it just tells Windows "This space over here where there used to be a file? You can put something else there, if you like."

It's kind of like moving out of an apartment by only taking your name off of the door. Until someone moves in and replaces with their own, all your stuff is still inside and available to anyone who knows how to look for it.

That's where the concept of "secure delete" comes in. A secure delete overwrites the data in the file when the file is deleted. This takes longer, of course, as it has to actually go access the entire file, but it ensure that the data is no longer accessible to the casual observer. It's kind of like making sure your apartment is empty - or at least full of stuff that isn't yours - before leave.

Unfortunately simply overwriting one file with another does not do this. The problem is that you can't control where the operating system is going to write the data. Depending on how the copy is implemented it may copy the data to a new file on the hard disk, delete your old file, and then rename the new file to the old file. (A very common technique.) Note the "delete" in the middle - that's not a secure delete. Your data is still out on the hard disk.

The bottom line is that, yes, a secure delete utility is probably what you need. It will ensure that the actual sectors on the hard disk that your file occupied are overwritten with other data.

Our friends over at SysInternals have such a utility for free: SDelete.

Alternately, there are also "free space wipers" that will simply write data to all the free space on your hard drive. This removes the data from any and all files that have been permanently deleted. As it turns out, SDelete will do this as well.

Now, there's one more step we need to look at before we say we're done, and this is where your level of paranoia, and the sensitivity of the data, come into play.

Because of the way magnetic material on hard disks works, it might be possible to actually recover data that has been overwritten. It's most definitely not trivial, and often requires special tools and techniques (and often a fair amount of money), but it sometimes can be done.

The solution is to ensure that the secure delete utility has the option to overwrite the data multiple times. (It can actually involve much more than that, based on things like physical disk configuration and disk head movement.) By writing over those soon-to-be-free sectors multiple times, the original data is well and truly gone.

Article C2766 - August 23, 2006

A version of this article that can be republished without cost is available at ArticlesByLeo.com terms).
Leo Leo A. Notenboom has been playing with computers since he was required to take a programming class in 1976. An 18 year career as a programmer at Microsoft soon followed. After "retiring" in 2001, Leo started Ask Leo! in 2003 as a place for answers to common computer and technical questions. More about Leo.

Not what you needed?

Recent Comments
29 Comments

For years the Cygwin utilities have contained shred.exe with, a.o., following options which may inspire:
- change permissions to allow writing if necessary
- Overwrite N times instead of the default (25)
- get random bytes from FILE (default dev/urandom)
- shred this many bytes (suffixes like K, M, G accepted)
- truncate and remove file after overwriting
- add a final overwrite with zeros to hide shredding

Posted by: Roger at December 5, 2009 12:45 PM

Thanks for sharing
PS: it's always good to have alternative so if it didn’t worked you can give a try SafeDeleter tool - i was using it for some time
http://www.safedeleter.com

Posted by: kbman at January 13, 2010 12:27 AM

Can I go into the history, even though it has been deleted, and find out what the history on the computer was?

Posted by: michelle at June 25, 2010 10:00 PM

ComputerWorld reports (March 7, 2011) that recovering data from both SSD drives and flash drives is incredibly easy even after being overwritten.

This article requires you to sign up. But it is harmless to do so. Remove the check marks from both boxes and you will not get any additional mailings. At least that is my experience.

This article is scary and should be required reading.

http://www.computerworld.com/s/article/355159/SSD_Security_Issues_Surprise_Experts

Posted by: Steve at March 16, 2011 9:34 AM

@Yeppers
The secure delete utility by Sysinternals, mentioned in the article, will do the job. It is a Microsoft program. Why doesn't MS include it in Windows is anybody's guess.

Posted by: Mark J at August 25, 2011 2:54 PM
Post a comment on "Secure Delete: what is it, and do I need it?":





Remember Me?

(You may use HTML tags for style)

Before commenting, please...

  • READ THE ARTICLE. A comment that shows you didn't will be deleted and ignored.

  • Comment only on the article. Use the search box at the top of the page if you have a question about something else.

  • NO PERSONAL INFORMATION in the comment. No email addresses. No phone numbers. No physical addresses.

  • Anything that looks the least bit like spam will be deleted. Links to unrelated sites or links that appear to be primarily promotional will be deleted, or the comment will be deleted.

  • Don't ask me to recover lost passwords or hacked accounts. I can't. Those comments will be deleted.

  • I can't respond to every comment. And I can't vouch for the accuracy of others who do.

Please wait. Your comment is being processed ...