Scrub free disk blocks

Marko Vojinovic vvmarko at
Sun Aug 29 06:46:49 UTC 2010

On Sunday, August 29, 2010 01:32:34 James McKenzie wrote:
> One thing is that if you expect the police on your doorstop, you are
> screwed anyway.  There is NO truly secure method, other than complete
> pulverization, to destroy disk data.

Whenever I see a statement like this (and this isn't the first time), I get 
baffled over and over with how is this possible.

Starting from the premise that every hard disk has in principle limited 
capacity to store data, one can always fill it up completely, then rewrite it 
completely again. I see no way of the old data being recoverable, because this 
is in contradiction with the fact that the disk was filled up completely two 
times. The old data has to be destroyed in order to make room for new data. At 
least as far as I can understand it.

OTOH, if the premise is false, it means that I can fill the disk with arbitrary 
amount of information and have it all recoverable in principle. That doesn't 
sound very reasonable, because the disk is made ultimately of a finite number 
of particles, and can thus be in finitely many different states. I see no way 
how a piece of metal can hold infinite amount of information.

Or let me put it more bluntly --- take a cup and fill it up with oil. Then try 
to fill it with the equal amount of water. The oil is going to overflow, and 
will be lost beyond any chance of recovery from within the cup. I see no way 
to avoid that. The structure of information storage on a hard disk is 
technically more intricate, but ultimately obeys the same principle.

So am I missing something?

Best, :-)

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