[plug] wiping a hard disk clean

[Peter F Bradshaw]

On Tue, 11 May 2004, Mark O'Shea wrote:

> On Tue, 11 May 2004, Russell Steicke wrote:
> > On Tue, May 11, 2004 at 03:59:26PM +0800, Ben Jensz wrote:
> > > Hey all,
> > >
> > > Just curious if anyone has any suggestions for something to run under
> > > Linux to completely wipe all data from a hard disk, irrespective of
> > > filesystem etc?  Some of the disks I'm looking at wiping may have
> > > Windows partitions and some have Linux partitons.
> >
> >  dd if=/dev/zero of=/dev/hdXX bs=1M
> >
> > > Basically such that you couldn't retrieve the data even if you tried hard?
> >
> >  dd if=/dev/random of=/dev/hdXX bs=1M
> >
> > But this will block waiting for random data quite frequently, so use
> > /dev/urandom if it's not really critical.
> >
> > Increasing the buffer size from 1M could be useful to increase the
> > speed, too.
> >
> Also remember that the hard drive is a magnetic medium, so when you write
> something to it you are only actually changing the orientation of *most*
> of the stuff on the disk, some gets through unaltered and this is how
> people with the $$$s can get at the data.  I read somewhere once that
> writing random data to the whole disk three times was considered to be
> good enough to stop all but those with much more time and money than sense
> (like US government agencies).
>
> The idea of melting the disk sounds the most fool proof if you really
> don't want anyone seeing your data...
>
>

I guess it depends on how paranoid you are. It used to be that government
agencies that worry about such things used to consider 16 flux revesals
sufficient. Now they will not specify. Only physical destruction will do.

I've been told that recovery 4 levels down is quite cheap and easy. I have
no first hand knowledge of this. But Linux's file wiping program overwrites
about 32 times.

Here's a paper:

http://wipe.sourceforge.net/secure_del.html

It includes this passage: "When all the above factors are combined it
turns out that each track contains an image of everything ever written to
it, but that the contribution from each "layer" gets progressively smaller
the further back it was made". Plainly there is some limit - Quatum
Mechanical if no other.

Cheers

-- 
Peter F Bradshaw, pfb at exadios.com, ICQ 75431157 (exadios).
http://www.exadios.com
PGP public key at http://www.exadios.com/public_key.html
"I love truth, and the way the government still uses it occasionally to
 keep us guessing.  " - Sam Kekovich.