How to troubleshoot a frozen boot sequence

[Billy Newsom]

I am doing a test run on a production server. It has 2 hard drives.

ad0 (mounted on /disk250 in a single slice plus SWAP)
twed0 (mounted on / /var /usr and a SWAP)

The twed0 is a hardware mirror and my main drive.
ad0 is just for backups.

What the issue is, and you probably know where I'm heading. The boot process 
freezes if I remove the ad0 (to test a drive failure condition)

It freezes after saying:
BTX boot loader.... etc.

FreeBSD/i386 bootstrap loader 1.1
It spins for a second, then stops... unless I have ad0 in the computer.
/boot/kernel/kernel text=0x7b03a0 data=0xcdee0 /

And it never gets to the boot menu.

So:

1. Should I put a new boot0config on the twed0 drive? If so do I boot from a 
CD to do that?

I need to potentially do something also to my disk labels and my fstab so that 
I don't boot to single user mode if drive ad0 fails. I haven't done this exact 
type of thing before, so I am looking for a little help.

my fstab:
/dev/ad0s1b             none            swap    sw              0       0
/dev/twed0s1b           none            swap    sw              0       0
/dev/twed0s1a           /               ufs     rw              1       1
/dev/ad0s1d             /disk250                ufs     rw      2       2
/dev/twed0s1e           /tmp            ufs     rw              2       2
/dev/twed0s1f           /usr            ufs     rw              2       2
/dev/twed0s1d           /var            ufs     rw              2       2
/dev/acd0               /cdrom          cd9660  ro,noauto       0       0


I tried to read the MBR from the twed0 drive, and the program couldn't read 
it. The one from the ad0 drive is readable and I saved a copy of it.