[CentOS] Utility to zero unused blocks on disk

Tue Feb 9 17:04:37 UTC 2016
Lamar Owen <lowen at pari.edu>

On 02/08/2016 07:38 PM, Always Learning wrote:
> I unscrew the casing, extract the disk platter(s), slide a very strong
> magnet over both sides of the platter surface then bend the platter in
> half.
> How secure is that ?
Actually, while there is some good security to that it's not for the 
reason you might suspect.  Just taking the platters out is enough, for 
most drives, unless you have the proper precision jigs necessary to get 
them properly realigned.

But a static permanent magnet can leave a lot more than you think; 
degaussing is almost always done with alternating magnetic fields and 
exponential decay.  The exponential decay is the key, and a proper set 
of passes with a permanent magnet taken at roughly 
exponentially-decaying heights will do the job (screwdriver 
demagnetizers have steps in their casing to do this).  An AC degausser 
needs to be kept on while removing from the media to fully degauss (or 
you end up with more residual magnetism (hysteresis) than can be 
overcome by the heads).

Piranha solution does a number on all known magnetic media (and skin), 
but bringing the media up to transition temperature will also 
effectively degauss.  So melting it down is guaranteed, and fun (but 
remove the printed circuit board and all other plastics first......).