machine won't start

[Carsten Mattner]

On Wed, Jul 4, 2012 at 8:07 PM, Matthew Dillon
<dillon at apollo.backplane.com> wrote:
>
> :Thanks Matt for the explanation and tip.
> :
> :It did of course hang when I tried to DEL into the BIOS.
> :What worked is pulling out the sata connector, entering
> :the BIOS putting it back and then detecting the disk.
> :Interesting the auto detection then worked. I've explicitly
> :set it to LARGE and now I can boot a rescue cd.
> :
> :How many bytes should I zero out for the disk to be
> :"normal" again? 512bytes? 4megs?
>
>     The BIOS is basically just accessing the slice table in the first
>     512 bytes of the disk.  If I want to completely wipe a non-GPT
>     formatted disk I usually zero-out (with dd) the first ~32MB or so
>     to catch both the slice table and the stage-2 boot and the disklabel
>     and the likely filesystem header.
>
>     Destroying a GPT disk requires (to be safe) zero'ing out both the first
>     AND the last X bytes of the physical media to also ensure that the
>     backup GPT table is also scrapped.  Again, to be safe I zero-out around
>     32MB at the beginning and 32MB at the end w/dd (if it's GPT).

How do I tell dd to delete x bytes at the end of device? Negative values?

>     This will effectively destroy everything on the disk from the point
>     of view of probing, so please note that these instructions are NOT
>     going to leave multi-OS installations intact.