Scrub free disk blocks

Marko Vojinovic vvmarko at
Sun Aug 29 19:21:31 UTC 2010

On Sunday, August 29, 2010 09:53:48 Michael D. Setzer II wrote:
> >   Marko Vojinovic <vvmarko at> wrote:
> > > Starting from the premise that every hard disk has in principle limited
> > > capacity to store data, one can always fill it up completely, then
> > > rewrite it completely again. I see no way of the old data being
> > > recoverable, because this is in contradiction with the fact that the
> > > disk was filled up completely two times. The old data has to be
> > > destroyed in order to make room for new data. At least as far as I can
> > > understand it.
> > 
> > At least at one time it was possible because the data is stored in a
> > region and when overwriting the region you don't hit the same spot every
> > time. With the right equipment you could see these areas and tell what
> > data had been written in that spot in the past.
> > 
> Recalling a presentation at Defcon 2006, the space between tracks would
> contain information that could determin what was there before a format
> operation.  A DES level wipe required writing 7 different patterns to every
> sector to make this practically impossible.

Ok, so if I read this correctly, after cca 7 rewrites of the whole disk with 
random contents, there is quite high probability that the original data is 
gone beyond any recognition ability, no matter how high is the budget of your 
favorite spy organization.

So if you want to be on a safe side, fill up the whole disk from /dev/random 
over and over 20 times, and the original data will be completely gone. Even 
for NSA & friends. :-)

Best, :-)

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