On Wednesday 02 September 2009 22:39:24 you wrote:
On 02Sep2009 22:17, Marko Vojinovic <vvmarko(a)gmail.com> wrote:
| On Wednesday 02 September 2009 21:32:32 Dean S. Messing wrote:
| > I have a terebyte sata drive that I need to securely wipe clean.
| I have always wondered about this, why not just do a rm -rf * on the
| drive, then put one big file on it (some divx movie or such), and copy it
| over and over under different names until the drive space gets exhausted
| completely? This can easily be scripted, and I believe it would work as
| fast as possible for a drive of given capacity.
Copying /dev/zero is a fast way to get an arbitrary amount of data (my
standard anecdote involves emptying it, which I did once on an ancient
system). It will be faster than copying a real file since the "read"
part is free.
You are right, zeroing is faster of course. I mentioned a dvix movie just to
make the data written more random than all-zeroes, which might be more secure,
but the end result is the same, I guess. :-)
The purpose of shred is to rewrite the data many times with random data,
since it is technically possibly to read "old" patterns from the drive
with the right (expensive and special) hardware.
This is the part that puzzles me. Let's give it a following thought
experiment. Suppose I have all that state-of-the-art expensive and special
equipment at my disposal, and unlimited free time. So I fill the drive with
data1, zero it out, fill it with data2. Are you saying that
I can use the equipment to recover the old layer of data1 (all or some part of
it)? Then I could zero the drive again, fill it with data3. Can I use the
equipment to recover both data1 and data2 layers which have been deleted?
Suppose I repeat the process arbitrarily many times. At some point data1 layer
would have to be lost completely, since otherwise it would mean that there is
a way to read and write infinite amount of data on the drive, which is
So the question is: if you suppose I have in my possession a yet-to-be-
invented-most-expensive-CIA-NSA-dream-about-it-machine for data recovery, how
many times should a typical drive be zeroed over and over in order to destroy
that first layer of sensitive data beyond any chance of recovery, even in
Given that I know so little about modern hard drives, I can only guess, but I
guess the number of such rewrite-cycles is ridiculously small, like 3 or maybe
4 top. It would need a serious scientific study to convince me that it needs 5
times to do it.
So what's all the fuss and hype about deleting drives, then? Create a script
to zero out (or random out) the drive four times, let it run for a week, and
be done with it. There should be some extremely serious arguments to convince
me that this would not be completely effective on any drive.