Whilst performing updates on Ubuntu server 16.04 we see the following warning. (Accepting it is a warning, it is none-the-less annoying).
Setting up initramfs-tools (0.122ubuntu8.3) ...
update-initramfs: deferring update (trigger activated)
Processing triggers for initramfs-tools (0.122ubuntu8.3) ...
update-initramfs: Generating /boot/initrd.img-4.4.0-38-generic
W: mdadm: /etc/mdadm/mdadm.conf defines no arrays.The warning is displayed whenever there’s no ARRAY line in mdadm.conf
The simplest fix for this warning is to add the following line to the end of /etc/mdadm/mdadm.conf
ARRAY <ignore> devices=/dev/sda