Scrub free disk blocks

Michael D. Setzer II mikes at
Sun Aug 29 08:53:48 UTC 2010

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 at>
To:             	Marko Vojinovic <vvmarko at>
Subject:        	Re: Scrub free disk blocks
Copies to:      	users at
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> On Sun, Aug 29, 2010 at 07:46:49 +0100,
>   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.
> 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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  Michael D. Setzer II -  Computer Science Instructor      
  Guam Community College  Computer Center                  
  mailto:mikes at                            
  mailto:msetzerii at
  Guam - Where America's Day Begins                        
  G4L Disk Imaging Project maintainer
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