On 29 Aug 2010 at 3:16, Bruno Wolff III wrote:
Date sent: Sun, 29 Aug 2010 03:16:28 -0500
From: Bruno Wolff III <bruno(a)wolff.to>
To: Marko Vojinovic <vvmarko(a)gmail.com>
Subject: Re: Scrub free disk blocks
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On Sun, Aug 29, 2010 at 07:46:49 +0100,
Marko Vojinovic <vvmarko(a)gmail.com> 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.
I have heard that with the current generation of disks this is no longer
practical. But practical is mostly defined by what your budget is; so if the
data is valuable enough, it is potentially recoverable.
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.
I don't do that level of wiping disk, but do use scripts to clear the unused
space before doing disk/partition images. Makes a huge difference in the
image size, since zeroed out sectors compress to almost nothing in the
image file. Did an image of an 80GB disk after a full install of Fedora, and it
made a 12GB image file. After clearing the image was only 2.5GB.
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