W: mdadm: /etc/mdadm/mdadm.conf defines no arrays. January 21, 2018UbuntuPF Edit /etc/mdadm/mdadm.conf and, after # definitions of existing MD arrays add ARRAY <ignore> devices=/dev/sda As seen on https://askubuntu.com/questions/834903/i-dont-have-a-raid-but-get-the-warning-mdadm-conf-defines-no-arrays