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Re: Erasing a hard disk easily
From: Valdis.Kletnieks () vt edu
Date: Wed, 14 Jul 2004 23:46:42 -0400

On Wed, 14 Jul 2004 00:44:47 EDT, Valdis.Kletnieks () vt edu said:

DOD 5220-22M says:

d. Overwrite all addressable locations with a character, its complement, then a 
random character and verify. THIS
   METHOD IS NOT APPROVED FOR SANITIZING MEDIA THAT CONTAINS TOP SECRET INFORMA-
   TION.

Hmm... "all *addressable* locations".  Looks like our spooks aren't all THAT
worried about their spooks reading stuff out of the spare and bad blocks. And
that makes sense, really, for the case of a basically working drive. Let's work
it through:

1) 99.8+% of the blocks on a disk are "good" and "addressable", and get wiped
as per (d) above.  No problem.

2) Originally spare blocks that have been allocated as replacements are
"addressable", and nailed above in (1).

3) Still-spare blocks aren't addressable, but we don't care because we
can't have written data to them - they're still spares.

4) Bad blocks may indeed have data - but they're bad blocks.  If the drive was
able to read them on its own, they'd not be bad, right?  As a result, an
adversary will probably not be able to read any data on those blocks without
some extreme technical assistance in a clean-room environment.  Also,
the number of bad blocks is some very small percentage of the total, so
there's a very low chance that any *really* critical data managed to land
on a bad block.  (This is assuming that the heads and servo track and
all that are basically OK, so the bad block is an actual media flaw).

Of course, this analysis only applies to mostly-working drives that can
still talk to most of the oxide - if you've lost the servo tracks or other
similar failure, then you still need to destroy the drive.  If half the
drive is unreadable because the servo tracks are shot, that means that
there's potentially multiple gigabytes of data easily recovered in a
clean-room setup....

Of course, if the drive is totally shot, it's just basic kindness to the
next owner to take it out and put 10 or 15 large holes in it with a drill
press (and it's fun, too :)

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