According to Tucker, R., SrA, 28CS/SCSNS:
>A small postive or negative current gets sent to the disk through the heads,
>charging the surface of the disk. Like ALL electrical currents, it flows in
>a wave pattern (frequency). The overwrite also flows in the same wave
>pattern, BUT NOT IN THE EXACT LOCATION AS THE ORIGINAL WAVE PATTEERN.
>Approximately 40% of the waves match after the first overwrite, leaving
>about 60% you could still bring back, provided you have the right software
>(some cases hardware is also necessary...I'm not the REAL expert on this,
>but I have seen how OSI does this.) I forget the actual algorythm on how
>much gets erased at each pass, but I do remember on the chart that after the
>first pass, approx. 60% could still be recovered, and it went all the way
>down to after the 99th pass, approx. .06% could still be recovered.
Oops, nice theory but totally ignores the head positioning slop in the
mechanics of the system. If you can rip the platters out and run them
under a suitable setup then you can pick up the tracks out of the slop
and get the data back. This why _very_ paranoid people (aka security
officers) will not accept the overwriting of the hard disk - you
cannot _prove_ that all the data has been overwritten. Sure they may
relent and downgrade the classification of the hardware to a lower
level but that box must still be secured to the highest level of the
data that was stored on it. Well, that's the way it works here in
Australia, I would imagine it is not too much different elsewhere.
>ANother thing you can do, if it's possible to separate your disk (the little
>case with the platters) from the controller board (or anything else you
>don't want zapped) is to run it through a degausser. Now on this one you'll
>have to look up the length of time, Oehrsteds (sp?), and for what
>coercivity, etc. as they change from HD to HD.
Bzzzt wrong - *if* you manage to degauss the sucker at all then you have
just stuffed the hard disk totally. All the modern hard disks I have
seen use voice coil head drives which implies that one of the platter
surfaces is devoted to disk postitioning servo information. Ever
wondered why disks have an odd number of recording surfaces? This is
the reason why, the servo is on one of these surfaces. Degaussing the
servo surface implies that the disk drive electronics will no longer
know where the f*ck the head is which makes the disk useless.
Brett Lymn, Computer Systems Administrator, AWA Defence Industries
"It's fifteen hundred miles to Ankh-Morpork" he said. "We've got
three hundred and sixty three elephants, fifty carts of forage, the
monsoon's about to break and we're wearing ... we're wearing ... sort
of things, like glass, only dark... dark glass things on our eyes..."
- Terry Pratchett "Moving Pictures".