unable to write data to disk ad0

[László Nagy]

  Hello,

I have a FreeBSD 6.0 system on i386. I would like to add a new label on 
my main FreeBSD partition (I have about 30GB free space left).
If I try to add a new slice (ad0s1g,  mount point: /shares 
size:20031MB,  UFS2+S) then I get this message:

unable to write data to disk ad0

I also tried to do the same with the bsdlabel utility, but it has so 
many options, and the examples given in

http://www.freebsd.org/cgi/man.cgi?query=bsdlabel&sektion=8

are too confusing to me. I do not want to accidentally overwrite my good 
partitions.
Can somebody tell me a simple way to add a new label? Then I guess I 
need to format it with

newfs /dev/ad0s1g

and finally mount it. Please help.

   Les