won't boot after 8.0-RELEASE upgrade

[Tom Worster]

after running freebsd-update -r 8.0-RELEASE upgrade my system won't boot. it
gets stuck on mountroot and i can't find the magic word it wants.

the system used to have two sata drives /dev/ad4 and ad6. they were
partitioned and sliced using the deafaults that sysinstall suggested.

at the boot prompt, lsdev says:

disk devices
  disk0: BIOS drive C:
    disk0s1a: FFS
    disk0s1b: swap
    disk0s1d: FFS
    disk0s1e: FFS
    disk0s1f: FFS
   disk1: BIOS drive D:
    disk1s1a: FFS
    disk1s1b: swap
    disk1s1d: FFS
    disk1s1e: FFS
    disk1s1f: FFS

which looks right, although i'm not familiar with the "disk" nomenclature.

entering ? at mountroot mentions ad4 and ad6.

geom_mirror was being used.

i've tried saying "load geom_mirror" and/or "enable-module geom_mirror" at
the boot prompt. neither made any difference.

nothing i've said to mountroot works:

ufs:/dev/ad4s1a
ufs:/dev/ad6s1a
ufs:/dev/mirror/gm0s1a
ufs:/dev/disk0s1a
ufs:/dev/disk1s1a

does anyone know the magic word? i'd be very grateful.

tom