Mailinglist Archive: opensuse (2459 mails)

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Re: [opensuse] scrathsing old harddisks
  • From: "Greg Freemyer" <greg.freemyer@xxxxxxxxx>
  • Date: Thu, 27 Mar 2008 15:52:47 -0400
  • Message-id: <87f94c370803271252tb8899cejf7f7c7e4791f1235@xxxxxxxxxxxxxx>
On Thu, Mar 27, 2008 at 3:21 PM, Sam Clemens <clemens.sam1@xxxxxxxxx> wrote:
Greg Freemyer wrote:
> On Wed, Mar 26, 2008 at 11:33 PM, Sam Clemens <clemens.sam1@xxxxxxxxx>


You would be amazed at what can be accomplished with scanning
>> electron microscopes (due to the fact that the path of an
>> electron is effected by magnetic fields.). From what I
>> understand, due to hysteresis effects, a track starts out
>> at 'full width', but each time a magnetic field is reversed,
>> a "tail" is left on each side. Apparently, these residual
>> fields can be used to reconstruct data which was previously
>> overwritten.
> I would love a true reference (from the last 15 years). I have spent
> many hours looking into the question. The best I have seen is people
> claiming they can recover a bit here and bit there from modern drives.
> Not even any full bytes.
> I have a NIST document that says labratory based recovery of data is
> impossible for disk drives 20GB or larger if the have been overwritten
> with a single pass of data. ie. Any data including all zeros.

That's nice to know! Probably has something to do with the
higher data densities (bits / mm^2)

The new drives have a very low signal to noise ratio and they use a
multi-bit encoding scheme. Think about a magnetic field that can
point in any of 16 directions to record a single nibble. I'm not sure
that is what they are doing, but it is no longer one bit at a time.
It is a more complex system. In fact they have to have DSPs on board
to pull the data out of the noise.

Once you overwrite even once there is simply not enough signal to
figure it out what used to be there. And if you magically do figure
out that a particular nibble previously had values of 3, 6, and 7,
there is no way to know which of those was recorded in what order.

Greg Freemyer
Litigation Triage Solutions Specialist
First 99 Days Litigation White Paper -

The Norcross Group
The Intersection of Evidence & Technology
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