fdisk on mounted disks ?

[Luigi Rizzo]

the other day i was trying to create a new freebsd slice on the
same disk i booted from (on a freshly updated RELENG_6) and both
fdisk and sysinstall failed with some error (i think they could not
write on the disk, or something similar).
boot -s did not seem to help, either.

I remember vaguely some time ago a bit of discussion
on this topic, but cannot recall the outcome nor any
keyword to retrieve the relevant messages.

So, does anyone know the trick to be used to modify
the slices on a mounted disk ? Or do i have to boot
from a different media ?

cheers
luigi