System unbootable after upgrade to FBSD 7

[Gerrit Kühn]

Hi,

I just upgraded my first system from 6-stable to 7. installkernel and
installworld went well. However, the system won't boot anymore. :-(
The bootloader still appears, grabs kernel and modules, followed by

Can't work out which disk we are booting from.
Guessed BIOS device 0xffffffff not found by probes, defaulting to disk0:

panic: free: guard1 fail @ 0x6c23c
from /usr/src/sys/boot/i386/loader/../../common/module.c:959

press any key to reboot



Maybe I did something stupid during the update, though I cannot imagine
what it might have been. Any hints how to get my system back online?


cu
  Gerrit