[unisog] Wiping hard drives before computer transfer

Daniel Feenberg feenberg at nber.org
Fri Nov 25 23:22:07 GMT 2005

On Fri, 25 Nov 2005, Carl Miller wrote:

> In regard to
> http://www.dshield.org/pipermail/unisog/2003-January/010022.php
> "Gutmann explains that when a 1 bit is written over a zero bit, the
> "actual
>  effect is closer to obtaining a .95 when a zero is overwritten with a
> one,
>  and a 1.05 when a one is overwritten with a zero"."
> Actually, he compared over-writing both a 0 and 1 with a 1, not as you
> say
> "a 1.05 when a one is overwritten with a zero".
> He desribed it all in terms of over-writing with a 1.

Thank you. A corrected and enlarged version of my essay "Can Intelligence
Agencies Read Overwritten Data" has been available at


for several years. I am always interested in additional information on the
feasibility of reading overwritten disk sectors, but so far I have not
heard from anyone claiming to have done so, or referring me to someone who
has done so. 

By coincidence, we recently had 24 scsi drives filled with confidential
data to discard, and ended up drilling the platters, simply because it was
much easier than mounting them once the computer that had controlled them

Daniel Feenberg

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