machine won't start

Matthew Dillon dillon at
Wed Jul 4 11:07:03 PDT 2012

: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).

    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.

					Matthew Dillon 
					<dillon at>

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