Update grub on mdadm RAID 1
Solution 1:
This was caused by /dev/sdb not being recognised anymore by Linux. It worked again after reboot but I am not sure what might have caused this.
However, I had to boot the server from rescue mode since grub was not correctly installed before.
These were the commands I used in grub rescue mode for booting the server:
set root=(md/1)
set prefix=(md/2)/usr/lib/grub
insmod normal
normal
linux /vmlinuz-<kernel-version>-generic root=/dev/md2
initrd /initrd.img-<kernel-version>-generic
boot