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):
ARRAY /dev/md3 level=raid1 num-devices=2 metadata=0.90 UUID=f41a4644:6b2a05f6:24055e35:10f66136
mdadm, metadata, format, ignored, various, versions, october, etc, conf, doesn, edit, arrays, array, dev, md, raid, num, devices, uuid,