[ale] destroy old drives

[Jim Kinney]

Heat above Curie point will always remove all data from all recovery methods.

On April 12, 2019 1:22:49 AM EDT, Alex Carver <agcarver+ale at acarver.net> wrote:
>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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-- 
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