• To view the status of an array:

  sudo mdadm --detail /dev/md0

• To mark a device as faulty:

  sudo mdadm /dev/md0 --set-faulty /dev/sda

• To remove a failed or spare device:

  sudo mdadm /dev/md0 --remove /dev/sda

• To add a device:

  sudo mdadm /dev/md0 --add /dev/sda

Ubuntu server 10.04.1 installer has a known bug that prevents new installation on software RAID system from booting (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.