Loader does not load after reinstall with previous mirrored disk attached

[Glen Barber]

Hello,

First off, this is not a real "I need help" post, though there are a
few questions that I'd like to get insight on, if possible - it is
more for informative purposes in the archives, should someone run into
this very random problem in the future. Additionally, I believe this
is far too unlikely this will happen to many others to consider it to
be a "problem", thus submitting a PR.

The story, cut down from its original 3 hour length:

For some time, I was dual-booting OpenSolaris and FreeBSD, on 500GB
SATA and 250GB IDE disks, respectively.  When I lost the need to run
OpenSolaris (Flash), I purchased a second 500GB SATA disk.  I added
the original SATA disk to gmirror, and synced the 250GB FreeBSD
install to the 500GB SATA drive.  Once that finished, I removed the
250GB disk, and installed the second 500GB disk, adding it to the
mirror.

All was fine, except I was not using all of the drive space available.
 This past weekend, I decided to create a slice to cover the remaining
250GB, initially to try ZFS, which failed for the following reason.
Here is where question 1 comes into play.

1.)  What is the correct way to completely break a gmirror setup,
replacing /dev/mirror/gm0 with the original /dev/adN configuration?
Is the assumption that once you initialize gmirror, you will never
need/want to go back to a single disk setup?  Is there a clean way to
fix boot0 so it does not look for a mirror, short of
'vfs.root.mountfrom' in loader.conf?

After rendering my system unbootable since I use a USB keyboard, and
was not able to do anything at the 'mountroot>' prompt, I decided it
may be a good time for me to finally upgrade to 64-bit, where I
purchased an additional 4GB RAM and burned a bootonly amd64 disc.  It
wasn't a big deal for me since I knew my data was mirrored and
available.  However, I did consider purchasing a PS/2 keyboard to fix
the system... But, let's be honest, who can resist an upgrade? :-)

Here is the real reason for my post (besides archival purposes).
Because I never disconnected the secondary disk (from the original
mirror), it caused the loader to hang upon boot - no errors, just a
blinking cursor on the top-left side.  One answer I received about
this, which makes perfect sense considering what I saw when I was
trying to figure out what was happening (until I disconnected the
second disk to avoid writing over it accidentally), is that the new
loader and the previous loader were in conflict at boot.  Once the
secondary disk was disconnected, the system booted fine.  Which brings
me to my second question:

2.)  loader problem?  In hindsight, I can expect the loader to hang
with two MBRs, but should this be expected?  If the loader did not get
"confused", any insight to what may have actually happened, since I
could not see any output whatsoever?

Regards,

-- 
Glen Barber