Check out this site that I found regarding mirroring disks on the fly… Debian-Administration.org.
This saved me some considerable grief when one drive in a customers server crashed… was able to replace the disk and re-establish the mirror without having to rebuild.
BTW – I know this is how it should be – I have just not had a hardware failure before where I needed to put it into practice.
Update: Once you have rebuilt the mirrors (in the event of a failure), make sure to recreate the /etc/mdadm/mdadm.conf file by using:
echo DEVICE /dev/sda* /dev/sdb* > /etc/mdadm/mdadm.conf mdadm --details --scan >> /etc/mdadm/mdadm.conf
You will need to change the DEVICE line to reflect the devices in your environment.