add device to raid

To create a RAID10 do the following
mdadm --create /dev/md0 --level=10 --raid-devices=4 /dev/sda /dev/sdb /dev/sdc /dev/sdd

to insert a new hard disc by replacing a broken one do
mdadm --manage /dev/md?? --add /dev/newpartition

to monitor your RAID and to send an email if a disc fails:
mdadm --monitor --scan --mail=office@wimmer-christian.de