A scientist back in '07 decided to figure this out and did a study on
drives current as of then as to the recoverability rate with
He found that recovering more than one bit at a given location, with a
single zeros-wipe was statistically impossible. Multiple passes, bit
patterns, 35-x erase, etc. was all further useless, the latter being
based on some speculation in a government paper from the 90's.
He then built a generic magnetic recovery microscope using drive heads
and a spin stand, and found that recovery of data that hadn't been
erased was *very* good. This is the interesting part...
ATA has had a 'secure erase' command for several years, which will erase
a drive in a couple hours to a degree that can't be recovered. The
trick is, the standard 'secure erase' only does blocks that haven't been
re-mapped by ATA bad-block reallocation. Some of the newer drives,
Seagates having been the first, have an enhanced version of the ATA
command that will also wipe out the re-mapped blocks.
So, go ahead and do your 35 different-bit pattern erases, and any data
you had in an ATA-remapped block is easily readable by the spin-stand
microscope because it never got re-written (the drive has declared those
blocks unwritable and re-mapped them). A relatively-quick enhanced
secure erase with hdparm will make the drive un-recoverable.
He also notes that with increasing densitites, bulk de-gaussers are
becoming unreliable. Drives shot or smashed can be pretty easily glued
back together and the intact surfaces read. This was all on a series of
posts on the SANS forensics blogs a while back.
I've been trying to get a list of the Seagate drives that have supported
the enhanced secure erase (the manuals don't specify), but I'm caught in
the hell of e-mailing with a rep who insists I just want to know how to
use Seatools for DOS. Anybody know someone in the right department at
The moral of the story is you should wipe your drives securely, if you
know you can, and donate them to a charity or re-purpose them. Save the
whales and all that.
Bill McGonigle, Owner
BFC Computing, LLC
Email, IM, VOIP: bill(a)bfccomputing.com
Social networks: bill_mcgonigle/bill.mcgonigle