(Don't know where this should go)
First off -
You should wipe the file system info on the drive(s) you want to use in the RAID. This is becuase you are going to format the RAID not the drive.
Please do consider you can RAID partitions or whole disks. It is advised to RAID the whole disk.
# dd if=/dev/zero bs=1M count=1 of=/dev/sdL#
Where L = letter and # = number
Now lets create the array
# mdadm --create --verbose /dev/md0 --level=# --raid-devices=# /dev/sdL#
This tells mdadm what drives you want to be in the array, and what RAID level you are going to use.
Once the RAID is created -
To get some detail on the RAID
# mdadm --detail /dev/md0
Now you need to actually format the RAID.
To remove a drive from an array (because it failed)
# mdadm --manage --set-fault /dev/md0 /dev/sdL#
# mdadm /dev/md0 -r /dev/sdL#
You must set it faulty before you can remove it!
To stop the array
I orignally used to to RAID 2 network block devices, which didn't pan out too well...but it did work!
How to remove an MDADM Raid Array, Once and For All! - http://ubuntuforums.org/showthread.php?t=884556
Managing RAID and LVM with Linux (v0.5) - http://www.gagme.com/greg/linux/raid-lvm.php
Growing a RAID5 array – MDADM - http://scotgate.org/2006/07/03/growing-a-raid5-array-mdadm/
The Software-RAID HOWTO - http://tldp.org/HOWTO/Software-RAID-HOWTO.html