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From: Julian Elischer <julian@elischer.org>
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Cc: freebsd-hackers@freebsd.org, Peter Steele <psteele@maxiscale.com>
Subject: Re: How can I force boot from alternate drive with boot.config?
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John Baldwin wrote:

> I think the unit number is largely ignored now.  The kernel used to believe it 
> for finding /, but the loader now reads /etc/fstab and sets a variable in kenv 
> to tell the kernel where to find /.

One of the most annoying "improvements" for the decade from my 
perspective..

because if / is not going to be where the loader thinks / is (because
that is a different F with a different /etc/fstab) then you need to
do lots of fancy footwork to undo the damage.

Oh and it's not terribly well documented how to get around it.
(that I've found)