Just a quick note because I couldn’t find much info about this on the web.
After installing Ubuntu 12.04 and creating a simple RAID 1 mirror everything looked great, until after a reboot. I was unable to assemble the array with an error message like the above. It turned out I had to remove the package mdraid and it magically started working again. Hope it helps someone.
Thanks to Jonik for the info.
Source: