newfs hangs while installing 6.1

[Klim]

Hi all,

I'm trying to install freebsd 6.1 and the installation hangs while
creating the root filesystem.
I have a Silicon Image 3512 RAID controller configured as a RAID 1 with
two 250GB maxtor drives. I'm using the amd64 distribution on a Pentium D
machine (with a asus P5B motherboard).
The last issued command appares to be:
newfs -O2 /dev/ar0s1a

Am I doing something wrong?
Has anybody observed such a behavior?

thanks
Francesco