This site is the Linux-raid kernel list community managed reference for Linux software RAID as implemented in recent 2.6 kernels. It should replace many of the unmaintained and out-of-date documents 'out there' such as the Software RAID HOWTO and the Linux RAID FAQ.
Sometimes, you even want GRUB to boot from the secondary hard disk in case the primary fails to keep the system up and running. This is however not as easy as one might think...
How to switch a Debian system on software RAID 1 (mirroring)
Hardware - 3Ware RAID-controller