Comment 3 for bug 538526

ceg (ceg) wrote :

From what you wrote the first ARRAY line should not be there. (You did not create it after creating the array.)

So I suspect maybe you called "dpkg-reconfigure mdadm" or
"/usr/share/mdadm/mkconf force-generate /etc/mdadm/mdadm.conf" and recreated your mdadm.conf already before you appended the line with the command you stated and rebooting.

So what you were seeing was also this:
Bug #136252 [->UUIDudev] mdadm.conf w/o ARRAY lines but udev/mdadm not assembling arrays. (boot fails)