[ale] erasing a hard drive

[Lightner, Jeff]

Yep - I started using /dev/urandom instead of /dev/zero when I read something that said if the overwrite data is predictable (e.g. all zeros) it might be easier to decide what real data it was trying to mask.





-----Original Message-----
From: ale-bounces at ale.org [mailto:ale-bounces at ale.org] On Behalf Of David Tomaschik
Sent: Friday, August 26, 2011 2:52 PM
To: Atlanta Linux Enthusiasts
Subject: Re: [ale] erasing a hard drive

On Fri, Aug 26, 2011 at 1:37 PM, Lightner, Jeff <JLightner at water.com> wrote:
> Of course you don't need a software at all.
>
> Erasing the data in a filesystem then unmounting the filesystem and running:
> dd if=/dev/urandom of=/dev/sda1/scramble bs=1M
>
> - will write random data on the partition (sda1 in the above - you can substitute other partitions/drive letters as necessary)
>
> However, any time I've seen this question come up someone invariably posts that it really isn't necessary for "modern drives".   They never quite say what their definition of "modern" is.
>
> In the past it was required to do something like the above multiple times but it really isn't necessary unless you're doing DOD or CIA work.

It is necessary to do at least one overwrite pass.  Recovering data
from a drive that has not been overwritten at all (e.g., mkfs or rm
only) is trivial.

The theory behind multiple overwrites has to do with the fact that
data isn't really stored in discrete chunks on the drive, and there
are algorithms that can theoretically recover/rebuild data from the
"edges" of the tracks.  However, this is substantially less of an
issue these days.  For one, those techniques were developed against
MFM and RLL encoded drives.  Drives using PRML and EPRML and
perpendicular recording are believed to be far more resistant to this
(as they have less area in which other traces are left.)

For a better explanation than I could write, see
http://www.cs.auckland.ac.nz/~pgut001/pubs/secure_del.html (especially
the epilogue).


--
David Tomaschik, RHCE, LPIC-1
System Administrator/Open Source Advocate
OpenPGP: 0x5DEA789B
http://systemoverlord.com
david at systemoverlord.com

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