[Gllug] Thoughts on secondhand server guide price

[Philip Hands]

On Wed, Jun 17, 2009 at 10:39:26AM +0100, t.clarke wrote:
> I seem to recall reading somewhere that once a disc has been overwritten you
> cannot recover old data from it using the drive's own heads/electronics.
> Instead you have to use a bit of fancy hardware (STM ?) to examine the platters
> directly and endeavour to figure out what was written to the platter prior to
> being overwritten.

I'm pretty sure that's not the case.

The thing to remember is that the signal on the disk is an analogue
signal, not a digital signal.

So, let's say that a field strength of 1 (of whatever unit) is meant to
be a logical 1, and 0 represents 0.  The normal controller will probably
treat anything over 0.6 as a 1, and anything under 0.4 as a zero.

That means that if a bit on a disk starts out with a zero in it, then
has a one written to it, and then gets a zero on top of that, it will
probably be closer to 0.3 that 0.0, so you can differentiate between a bit
that's never been set, and one that was recently set, just by adjusting
the thresholds on a normal controller.

Apparently you can often get quite a lot of data off a drive simply
by dropping the 1 threshold and or increasing the sensitivity of the
input circuit -- I'm not au fait with the exact procedure, but I'd
imagine that's a case of re-flashing the on-disk-controller firmware. so
no fancy equipment required.

> I also recall that overwriting a disc several times with different random data
> rather defeats that approach as well.

I'd agree with that.

> I would imagine that physical destruction of the disc is warranted only where
> the data is INCREDIBLY sensitive.

and in that case, you have to consider the chance that someone might
actually reassemble the bits, and use lots of fancy kit, so I doubt a
hammer is enough in that case.

Cheers, Phil.
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