Unfortunately the above tutorial is outdated for Ubuntu 12.04. It was written for 10.04.
Ubuntu 12.04 after a regular installation has 3 partitions: The main one, a swap partition, and an extended one. I don't know what the last one is used for!
So when I am executing
fdisk /dev/sdb I cannot convert the extended partition to a linux raid autodetect. The error message says that this conversion is invalid and that I should delete the partition first!
What should I do? My ultimate goal is to setup a raid-1 system with the disk that already has ubuntu fully setup and a new disk that is currently empty. The one disk must be the mirror of the other so I imagine all partitions of the second disk must match the partitions of the first disk and each one should have a corresponding raid-1 partition.