cat /proc/mdstat
sudo mdadm --detail /dev/md0
sudo mdadm /dev/md0 --set-faulty /dev/sda1
sudo mdadm /dev/md0 --remove /dev/sda1
sudo mdadm /dev/md0 --add /dev/sda1
Ubuntu server 10.04.1 installer has a known bug that prevents new installation on software RAID system from booting correctly (see http://ubuntuforums.org/showthread.php?t=1474950 for more details). One possible solution to get around this problem is to setup disk partition and software RAID using 9.10 installation CD before installing 10.04.