mdadm: metadata format 00.90 unknown, ignored.

This happens with various versions of older mdadm such as mdadm - v2.6.7.1 - 15th October 2008

It is all because an extra 0 in 00.90 in /etc/mdadm/mdadm.conf that it doesn't like (it doesn't seem to cause any problem except that message though):

Solution - Edit your /etc/mdadm/mdadm.conf and change 00.90 to 0.90 in your arrays:

ARRAY /dev/md3 level=raid1 num-devices=2 metadata=0.90 UUID=f41a4644:6b2a05f6:24055e35:10f66136