For the last couple of weeks we've been working on setting up a bunch of Dell C2100 servers with Ubuntu 10.04. These servers come with 2 x 500 GB internal disks that can be set up in RAID1 with the on-board RAID controller. However, when we did that during the Ubuntu install, we never managed to get back into the OS after the initial reboot, or in some cases GRUB refused to write to the array (I think it was when we tried 11.04 in a desperation move). To make matters worse, even software RAID via mdadm stopped working, with the servers going to the initramfs BusyBox prompt after the initial reboot. My guess is that it all stems from GRUB not writing properly to /dev/md0 (the root partition RAID-ed during the install) and instead writing to /dev/sda1 and /dev/sdb1. So we decided to install the root partition and the swap space on /dev/sda and leave /dev/sdb alone.
I started to look for articles on how to set up RAID1 post-install, when you have the OS installed on /dev/sda, and you want to add /dev/sdb to a RAID1 setup with mdadm. After some fruitless searches, I finally hit the jackpot with this article on HowtoForge written by Falko Timme. I really don't have much to add, just follow the instructions closely and it will work ;-) Kudos to Falko for a great resource.