[ale] destroy old drives

[Alex Carver]

No, the magnetic strength of the head itself hasn't changed because
there's still a minimum field required to flip the domain of the
material which is intrinsic to the material coercivity.  Instead the
head has changed technology from old wire-wound heads to metamaterials
that exhibit giant magnetoresistance.  The head is now physically much
smaller so it can confine the field into a tiny area while avoiding some
of the divergence that can overwrite adjacent tracks.  Remember that the
platter already has servo tracks buried within it which the drive does
follow to maintain positional tolerance so there's certainly more than
one layer of material present in the platter.

Even still, print through happens but it does take time.  It's also not
necessary for the domains below the surface to fully align.  The modern
hard drive is an exercise in digital signal processing more than using a
hysteretic circuit to detect the individual domains.  The domains are so
small and traveling so fast past the head that it's actually an spread
spectrum RF signal being transmitted from the read head.  Modern drives
don't read individual ones and zeros anymore, they read a waveform and
use statistical processing to recreate the bits on the other end.

The magnetic domain won't be fully erased with a basic rewriting
program, there will always be a residual field especially if those
particular domains are not rewritten very often.  So it's not impossible
to do some additional DSP magic and deconvolve what is written on the
surface from what is deeper in the layer, information that will subtly
alter the waveform.  We already rely on this technology in things like
the GPS system and other similar spread-spectrum/ultra-wideband devices.
 A determined actor could filter out the strong surface signal and
eventually recreate a good portion of the underlying signal.

On 2019-04-11 14:09, Jim Kinney wrote:
> Really? The only groups that want the data _that_bad_ have subpeonas.
> The other groups that can read around the holes already have your data.
> All you're really trying to do is make sure the drive is not usable for
> the basic computer bad guy.
> As areal densities have increased exponentially from 10M drives to 10TB
> drives in the same space, the size, and thus strength, of the magnetic
> domains has decreased exponentially. So the bleed over has also
> decreased. The transition to vertical magnetic domains  has made the
> crosstalk to the platter substrate nearly zero. Add in the platters are
> simply not magnetizable at all and there's basically no data bits
> anywhere possible except on the platter surface.
> bad ascii art:
> N-S   bit domain on surface___    platter surface
> S-N   induced bit domain subsurface
>   N  S   |    |        2 adjacent vertical domains 1
> 0  S   N____       platter surface
>   N - S     Induced data bits are just wrong! Now mix in the 2D spacial
> arrangement and which subsurface pole pairs with which other? No
> monopoles in magnetic media (yet :-)
> 
> 
> On Thu, 2019-04-11 at 13:31 -0700, Alex Carver via Ale wrote:
>> If someone really wants your data, holes don't matter.  The rest of
>> theplatter is still intact in that case and can have the data
>> extracted.
>> There's also no guarantee that Dban can write enough to be sure that
>> themagnetic domains are fully randomized deep in the platter.  The
>> longerdata sits statically on the disk  the more opportunity for the
>> surfacedomain to imprint on deeper domains (this is actually a
>> problem withmagnetic tape, magnetic data can print through from one
>> layer of tape tothe next layer when it's wound on the spindle).
>> A serious entity can perform a deep level scan of the platter
>> andretrieve the low level signal under the surface domains and see
>> previousdata.  The drive head typically isn't powerful enough to
>> write thatdeeply because it has to keep the tracks narrow.
>> On 2019-04-11 12:13, Steve Litt via Ale wrote:
>>> On Wed, 10 Apr 2019 22:11:42 -0400Jim Kinney <jim.kinney at gmail.com>
>>> wrote:
>>>> Dban advantage: it can be done across hundreds or thousands of
>>>> drivesbefore larcenous third party "shredders" physically touch
>>>> the drives.
>>>
>>> That's a good point.
>>> Doesn't dban take an hour or more? How many drives can I do with
>>> onecomputer? How long would it take to test whether each is really
>>> blank?
>>> What might be nice with 1000 drives to do is dban followed by
>>> drilling3 holes in each drive. I'd say each drive would take 1
>>> minute for 3holes, so it's about 2 days for one employee to drill
>>> the holes. Or,perhaps, one employee could both dban and drill the
>>> holes, drilling theholes while the next batch is dbanning.
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