I started getting this error whenever I restart my OS. I tried to erase and install Ubuntu Linux again and the installation freezes at "Running update-grub". Not sure if those two things are related.
I have a lenovo G500 and I run Ubuntu 18.04 LTS version on it.
Let me know if you need more details.
I think this message comes when you don't have hard disk arrays. While booting, mdadm checks for arrays and don't find any. So this message is annoying, but harmless.