freebsd stable - Mar 2010 - Geom not found: "gm0" / Failed to write sector zero

I just installed 7.3-RC2 amd64 on new server.

I created slice s1 (80GB on disk ad4 (500GB), then partitions for system 
(/, swap, /var, /usr, /tmp) by sysinstall.

After base install I created gmirror gm0 as usual (I did it many times).

Now I am no longer in datacenter and have only ssh access to this server 
and I need to create slice s2 with some partitions for data storage, but 
fdisk failed.

     fdisk -u /dev/mirror/gm0

At the end, I got this error:

     Should we write new partition table? [n] y
     fdisk: Geom not found: "gm0"
     fdisk: Failed to write sector zero

Fdisk failed even if I used

     sysctl kern.geom.debugflags=16


Question #1 - why 'Geom not found: "gm0"'?

Question #2 - is there any way to create slices + partitions on unused 
space if system is booted from this device?
Or is the only way to boot it from some LiveFS / fixit?


I found the same question on this list, but without reply
http://lists.freebsd.org/pipermail/freebsd-stable/2009-June/050855.html

I hope somebody can help / explain it.

Miroslav Lachman