problem creating slices and labels

[Renato Marques Sant`Anna Ferreia]

Hi,
I am using FBSD 5.1 release and I can't create slices or labels using sysinstall.
When I boot from the instalation CD everything works fine.
The message from sysinstal is "ERROR: Unable to write data to disk ad0!"
I need to do this in a remote computer, so i can't boot from the CD.
What am I doing wrong?