Question on shredding a terebyte drive

[Bruno Wolff III]

On Wed, Sep 02, 2009 at 13:32:32 -0700,
  "Dean S. Messing" <deanm at sharplabs.com> wrote:
> I have a terebyte sata drive that I need to securely wipe clean.  It

How securely? (I.e. what order of magnitude is the budget an adversary is
assumed to have?)

> The drive is capable of about 60MB/sec, but shred is only "shredding"
> about 25MB every 5 seconds according to its output.  Since the default
> number of passes is 25, this works out to about 5 days.

For many definitions of secure, one pass writing zeros will make the cost
of recovering any data beyond the benefit to your assumed adversaries.
Your biggest risk is probably going to be that you thought you overwrote
the disk but made a mistake and didn't (or only partially did).

Note that in most cases where the adversary is assumed to be able to afford
to try to recover spare blocks or use electron microscopes to try to figure
out what may have been written previously, you should be physically destroying
the drive (after wiping) rather than save a few bucks repurposing or selling
it.