Comment 10 for bug 1318568

I had this problem as well with software raid after recent upgrade to Xenial (and then after another update to Xenial packages to 2.02~beta2-36ubuntu1 — however, at about the same time (at least after the last reboot), I've also added lvm-cache to the root partition, and that seems to have been the actual problem.

Latest Xenial grub cannot boot from lvm-cached system/boot (they are one and the same for me) partitions.

Workaround: disable caching of the system partition. I'll probably move /boot out into separate partition as well, but thought I'd throw this out as well.