gmirror and booting one and/or the other of the twins, then rebuilding raid 1

[John Crawford]

Hello,

   I've found it helpful to take a raid 1 system,
break the raid, and then be able to boot to either
of the two system disks. Once I'm done with this
split personality development and testing, I choose
which of the two to keep, and then rebuild
my raid 1 using that "kept" disk as the master.

I can do this with bios controlled raid controllers
easy enough. I'd like to do it with a gmirror'ed
system. I'm using a procedure much like that described here
to build my gmirror raid on freebsd 6.2.
http://dannyman.toldme.com/2005/01/24/freebsd-howto-gmirror-system/

How could I play boot swap-a-roo on these twin
disks? That is without pulling drive cables for
data or power? I'd like to be able to boot either of the
two drives. FreeBSD see's my devices even if I disable
the drive "OFF" in bios, so that alone doesn't work.

I suppose I could use kernel.conf and "di ad0" or "di ad2"
to suppress drive hardware detection, but I'm hoping to
do something simple (with a few keystrokes) during
one of the boot stages to suppress one or another of the
drive detections. I don't recall how to disable a given
device during the an interactive boot procedure.

And then of course, I'd like to be able to re-raid based
on my preferred drive, repopulating the other.

I'm not certain how best to manage this.
Thanks for some ideas.
John