Comment # 42 on bug 935993 from Neil Brown

Hmm... I've seen something a bit like this recently.  I just worked around it
rather than digging it to find the real problem.  Maybe I should try harder.

Meanwhile, could you provide the following files or command outputs please?

/etc/fstab
/etc/mdadm.conf
/proc/mdstat
/boot/grub2/grub.cfg
/proc/mounts
/proc/swaps

blkid

That will hopefully give me enough that I can pick out the detail that is
particular to your setup.