[Bulk] Re: What's in my hard drive? How can I get rid of it?

jd1008 jd1008 at gmail.com
Wed Feb 18 03:51:18 UTC 2015

On 02/17/2015 06:02 PM, Ralf Mardorf wrote:
> On Tue, 17 Feb 2015 18:37:38 -0600 (CST), Valeri Galtsev wrote:
>> On Tue, February 17, 2015 5:30 pm, Daniel Feenberg wrote:
>>> On Tue, 17 Feb 2015, Michael Powell wrote:
>>>> jd1008 wrote:
>>>> [snip]
>>>> Remove the cover. Remove the platters. Smash all platters with large
>>>> sledge
>>>> hammer until all pieces are fairly small. Melt material with
>>>> oxyacetylene
>>>> welders torch. Repeat smashing with hammer. Soak for few hours in
>>>> hydrofluoric acid. Rinse and allow to dry. Grind material into a
>>>> fine particulate dust. Dispose of out the back of airplane while
>>>> flying or drop
>>>> into convenient nearby volcano. That might be good enough.
>>>> Send the electronic components to Kaspersky for analysis.
>>> I did once investigate claims that overwritten sectors could be read
>>> by sophisticated instruments and posted my results at:
>>>     http://www.nber.org/sys-admin/overwritten-data-gutmann.html
>>> In short - that is pure science fiction.
>> Interesting. I never saw this particular explanation. I have heard that
>> overwiritten data can be recovered (to significant extent). Due to
>> different reason. I lost the reference, so let me try to reproduce what
>> I've read (and that didn't and still doesn't offend my university
>> degree knowledge, physics was part of it):
>> If some information sits intact for years (that is without any change
>> in particular physical places on disk platters), then over time due to
>> some sort of aging magnetic domains become slightly different (in
>> size, maybe?) in places where magnetization is in one direction from
>> that in the other direction. If to overwrite that information, nothing
>> original becomes readable. If, however you magnetize the whole bulk of
>> platter in one direction, then the areas that were magnetized for very
>> long time in one direction have slightly different residual
>> magnetization from areas magnetized for long time in different
>> direction. This difference is much smaller that sensitivity of regular
>> drive magnetic heads and pre-amlifier. However, more sensitive
>> equipment is capable to detect that thus very significant portion of
>> information can be recovered (not 100% though). You can recover
>> information even if it was overwritten with some new junk, provided
>> this junk didn't sit there comparably long.
>> One still can fully wipe the information, even sitting there due to
>> those "aged magnetic domains" if one re-magnetizes platters in opposite
>> directions many times (over 1000) going deeply into hysteresis every
>> time. One time overwriting data is definitely not enough. Using some
>> 50-100 cycles may not be sufficient either (but already this number of
>> cycles becomes impractical with all software based destroying of
>> information).
>> Sorry about long e-mail. Sledge hammer and shredder are the best for
>> the purpose ;-)
> Actually criminal investigation departments seems to be unable to
> recover all the data that was deleted by a simple rm command, even on
> journaling file systems. Why is it recommended to mount read only, as
> soon as possible, if we lose data, to be able to recover that data?
> The NSA is able to recover all the data that was deleted all over the
> world even by a shred command on a non-journaling FS? If so, the NSA
> isn't willing to give hints against child molesters and other criminals,
> because the NSA is the watchdog of more important crimes? That's
> grotesque.
Well, you have to admit - they do have priorities :)
Self interest overrules all other interests.

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