machine won't start

[Carsten Mattner]

On Wed, Jul 4, 2012 at 7:31 PM, Matthew Dillon
<dillon at apollo.backplane.com> wrote:
>     Normally this issue can be fixed by setting the BIOS to access the
>     disk in LBA or LARGE mode.  The problem is due to a bug in the BIOS's
>     attempt to interpret the slice table in CHS mode instead of logical
>     block mode.  It's a BIOS bug.  These old BIOS's make a lot of assumptions
>     w/regards to the contents of the slice table, including making explicit
>     checks for particular OS types in the table.
>
>     I've only ever seen the problem on old machines, and I've always
>     been able to solve it by setting the BIOS access mode.
>
>     I've never, ever found a slice table format that works properly across
>     all BIOSs.  At this juncture we are using only newer (newer being 'only'
>     25+ years old) slice table formats (aka LBA layouts and using proper
>     capped values for hard drives that are larger than the 32-bit LBA layout
>     can handle).
>
>     Ultimately we will want to start formatting things w/GPT, but that opens
>     up a whole new can of worms... old BIOSes can explode even more easily
>     when presented with a GPT's compat slice format, at least as defined
>     by GPT.  Numerous vendors such as Apple modified their GPT to try
>     to work around the even larger number of BIOS bugs related to GPT
>     formatting than were present for the older LBA formatting.
>
>     I consider it almost a lost cause.
>
>                                                 -Matt
>

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?