On one perfectly good 40GB hard drive I own, I had one (and only one)
partition... an MBR (#1) partition of 12GB in size.  Within that 12GB
partition I had already installed a perfectly good, working copy of
FreeBSD 9.0-RELEASE.  This was working just fine.

I decided to set up the drive so that I could dual-boot to either
9.0-RELEASE or else to to 9.1-RC2, so I proceeded to install 9.1-RC2 into
a second partition (MRB #2) to which I allocated 16GB.  The install
went entirely normally, as far as I could tell.  At the end, I was
asked (as per normal) if I wanted to reboot to the new system, to
which I answered yes.  The system began to reboot.  As the initial
BIOS diagnostics ran, I removed the 9.1-RC2 install CD (as I normally
do at the end of an install).  A moment or two later, I saw the standard
notices from the BIOS:

    Verifying DMI pool data ............
    Boot from CD:
    Boot from CD:

(Note:  I _do_ have two optical drives in the system in question, so the
above BIOS message are entirely normal and customary for this system.)

After this, the blinking cursor sat underneath the letter "B" in the final
message above.

At this point I expected to see the normal and customary messages from the
standard FreeBSD MBR boot loader... you know... about F1, F2, etc., etc..
However at this point *nothing* else printed, and the system just hung
forever.  I then performed a processor reset and got the exact same result
and the exact same hang at the exact same point.  I then also tried power
cycling the machine and still got the exact same result.

I am filing this PR as critical/high because it sure seems that way to me.
My apologies in advance if it doesn't really qualify as critical/high.
I just hate to think of inexperienced FreeBSD users banging into this.
Me?  I can always find a workaround.  I just pity the FreeBSD users who
may not bwe knowledgable enough to do so themselves.

(I hope that this problem isn't just down to pilot error.  I don't think
that it is, and I can't imagine anything that I missed doing during the
second install.  I don't recall having been asked during that process
whether or not the FreeBSD MBR boot loader should or should not be
installed into the MBR.)

Fix: 

Beats me!  I'm not even 100% sure what's gone wrong, let alone how to
fix it.
How-To-Repeat: 
See above.

Take a bare drive, install FreeBSD 9.0-RELEASE onto it, using a single
MBR-type partition, and leaving enough unallocated room on the drive for
another install.  Reboot after this initial install to make sure you have
indeed installed a working system (and to make sure the FreeBSD MBR boot
loader is installed and working correctly).

Then install FreeBSD 9.1-RC2 into a second MBR partition.  Finish the
install, request a reboot, and then, as the BIOS is rebooting the system,
remve the install CD and otherwise allow the BIOS to contine with the boot
process.

Then just wait for the choke.  (It should arrive swiftly thereafter.)