Comment 0 for bug 1635049

mdadm does not attempt to start partial md devices (incremental assembly) during initramfs and can cause system to fail to initramfs prompt if rootfs on md.

http://askubuntu.com/questions/789953/how-to-enable-degraded-raid1-boot-in-16-04lts

Fixed in debian mdadm 3.4-2: https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=784070