I ran into an issue on a specific Mythbuntu 14.04 machine where the RAID was halting the system on boot-up giving the following error:
incrementally starting raid arrays mdadm: Create user root not found mdadm: create group disk not found incrementally started raid arrays
If CTRL-ALT-DEL was pressed, the machine would reboot like normal and the RAID would appear to be OK. After much searching I came across the cause and the fix of this MDADM issue.
The problem was that mpt2sas.ko was not loaded, and thus mdadm couldn’t see any disk. This in turn was caused by a packaging error: when apt was removing unneeded wlan packages like crda, it also auto-removed the linux-image-extra-$VERSION package, which actually owns mpt2sas.ko.
To ensure, that the module gets loaded before the raid modules, I also did a
echo mpt2sas >>/etc/initramfs-tools/modules
After that, reboot and all is fixed!