Well, in that case probably Dban will be sufficient for your needs.  If still going with dd I agree with the bs=1M switch.

Incidentally, a discussion with a friend this evening lead us to the decision that a good way (although involved) to erase a drive would be:

1) Open disk enclusure.

2) Use bench grinder to create aluminium powder from the top of the disk enclosure
3) find something rusty, (or another source of iron oxide) and create powder.
4) mix 50:50 by volume.
5) if available add some magnesium (left over guy fawks sparklers?)
6) pour powder over HDD spindles.
7) Light and step well back while the platters melt.  

I might try this in coming months and report my results ;)

Daniel

On Mon, Sep 2, 2013 at 6:30 PM, David Zuccaro <david.zuccaro@optusnet.com.au> wrote:

On Mon, 2013-09-02 at 17:45 +1000, Daniel Cross wrote:

> I guess it depends on your threat model, so rewinding a bit - who are
> you wanting to prevent accessing your data? This greatly informs the
> conversation.  The methods offered above, and in many of the replies
> thus far, assume a very sophisticated adversary.
>
> Daniel

Let us assume that the adversary is not a
three-letter-government-agency; not that I consent to
three-letter-government-agencies accessing any of my information.