On 4/4/2012 9:13 PM, NeilBrown wrote:

If the array was activated read-only, that suggests that 'mdmon' didn't run.
mdmon is a daemon which monitors the array and updates the metadata as
needed.  until it is running it is not safe for the array to be writable.
However it should be started automatically by mdadm.

That explains it... mdmon does not appear to have been added to the 
Ubuntu package.

How are you attempting to start the array?

mdadm --activate --scan