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If I wanted to completely and securely wipe a hard drive, could I simply write all ones or zeroes (or even just random zeroes and ones) over the entire disk to clean it? It'd probably take forever, but is my theory correct?

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No! Magnetic hard drives have "memory". Forensic tools will still be able to recover (part of) the files. –  Dennis Mar 26 '12 at 15:50
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Depends on you paranoia level, "average joe computer guy" a few write cycles should be more than fine, CIA/NSA/MI6/etc. thermite :) –  Lamar B Mar 26 '12 at 15:54
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@Dennis: That is just not true. See my comment on RedGrittyBrick's answer. –  Daniel Andersson Mar 26 '12 at 16:00
    
You may be interested in the conclusions of Security.SE on this one, particularly the top voted answer. –  Ninefingers Mar 26 '12 at 18:36
    
security.stackexchange.com/questions/8965/… is what I got for answers when I asked –  Canadian Luke Mar 26 '12 at 18:49

2 Answers 2

up vote 5 down vote accepted

That should suffice. There is a theory that you need to do this multiple times because of variations in tracking, however the threat countered by this seems largely theoretical.

You may find it easier to use a drill or a very large hammer.

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I do agree however that unless you are giving the machine away, selling it, or donating it to a charity, physical destruction is best. Besides, HD magnets make great toys and refrigerator magnets. –  EBGreen Mar 26 '12 at 15:57
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springerlink.com/content/408263ql11460147 - a reference to that the threat is purely theoretical. Not a single application of this technique has been documented as successful, even after just a single zero overwrite. See also h-online.com/newsticker/news/item/… for a summary. As data densities increase immensely, it gets even more unfeasible by the year. And the equipment that is supposed to be used is not cheap, to say the least. –  Daniel Andersson Mar 26 '12 at 16:00
    
@EBGreen, what tools? –  ultrasawblade Mar 26 '12 at 16:01
    
You are correct. When you put the question to me I went back and did some research. My statements were based on older articles that have since been clearly refuted. I retract my previous statements. –  EBGreen Mar 26 '12 at 16:09
    
Physical destruction doesn't mean that data recovery is impossible. (Unless you have all the pieces...). It's possible to reconstruct magnetic media, if you have quite a bit of patience.. (Impractical in most cases, but it's been done before...) –  Benjamin Schollnick Mar 26 '12 at 18:01

Once upon a time there was a man named Peter Gutmann who suggested that with the use of an electron microscope you might be able to figure out what was once written to a part of a hard drive that had been overwritten. That lead people to come up with policies like “you have to overwrite 7 times before it is safe to dispose of.” However, despite all the assurances I’ve heard that it can be done, nobody knows anyone that has actually done it. A couple years back I took a forensic class with Mike Murr from the SANS institute and he was talking about this very thing.Everyone knows someone that has done it, but nobody has done it themself.

As a general guide line, wiping the drive 3 times is the maximum wipe count that I will use. Beyond that is overkill, unless your are dealing with extremely sensitive data.

Any data recovered after 2 wipes, is probably impossible. 3 or 5, would make it virtually impossible.

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