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From: John Baldwin <jhb@freebsd.org>
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Date: Wed, 15 Aug 2012 11:16:21 -0400
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	(bigwig.baldwin.cx); Wed, 15 Aug 2012 11:52:25 -0400 (EDT)
Cc: Garrett Cooper <yanegomi@gmail.com>
Subject: Re: GPT boot from 2nd. disk fails
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On Wednesday, August 15, 2012 4:46:28 am Garrett Cooper wrote:
> On Wed, Aug 15, 2012 at 1:27 AM, Daniel Braniss <danny@cs.huji.ac.il> wrote:
> > hi,
> > this host has to disks:
> > sa0> gpart show
> > =>       34  976773101  ada0  GPT  (465G)
> >          34        128     1  freebsd-boot  (64k)
> >         162    4194304     2  freebsd-ufs  (2.0G)
> >     4194466   33554432     3  freebsd-swap  (16G)
> >    37748898  939024237     4  freebsd-zfs  (447G)
> >
> > =>       34  976773101  ada1  GPT  (465G)
> >          34        128     1  freebsd-boot  (64k)
> >         162    4194304     2  freebsd-ufs  [bootme]  (2.0G)
> >     4194466    8388608     3  freebsd-swap  (4.0G)
> >    12583074  964190061     4  freebsd-zfs  (459G)
> >
> > but no amount of magic will cause boot from the second disk, it will 
always
> > boot from the first disk.
> >
> > any insights?
> 
>     Use boot0cfg -s 5 (untested with GPT disks)?

Will not work with GPT disks.  They use /boot/pmbr to boot, not /boot/boot0.

If you can get your BIOS to explicitly boot ada1 from the start via a BIOS
setting, that should work.  Another option would be to break into gptboot's
prompt (similar to breaking into boot2) aud typing in 'ad1p2:/boot/loader' or 
some such.  If that works you should even be able to write that to 
/boot.config on ada0p2's filesystem.

-- 
John Baldwin