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Short backstory: I have never done anything with mdadm before and am in charge of maintaining an ubuntu 16.04 server with a software RAID 1 based on mdadm. Recently I updated grub and was prompted to select which disks grub should be installed on.
I chose to install it on both /dev/sda and /dev/sdb because I have read that it does not matter to mdadm and is better to install GRUB on both disks.

Currently I am trying to update the linux image, but I get the following error during the post installation script:

grub-probe: error: disk `mduuid/[uuid here]' not found.

The installation fails after this message.

This is the complete output of the post installation script:

Examining /etc/kernel/postinst.d.
run-parts: executing /etc/kernel/postinst.d/apt-auto-removal 4.4.0-72-generic /boot/vmlinuz-4.4.0-72-generic
run-parts: executing /etc/kernel/postinst.d/initramfs-tools 4.4.0-72-generic /boot/vmlinuz-4.4.0-72-generic
update-initramfs: Generating /boot/initrd.img-4.4.0-72-generic
run-parts: executing /etc/kernel/postinst.d/zz-update-grub 4.4.0-72-generic /boot/vmlinuz-4.4.0-72-generic
Generating grub configuration file ...
grub-probe: error: disk `mduuid/[uuid here]' not found.
run-parts: /etc/kernel/postinst.d/zz-update-grub exited with return code 1
Failed to process /etc/kernel/postinst.d at /var/lib/dpkg/info/linux-image-4.4.0-72-generic.postinst line 1052.

I have no idea how to fix this and can't seem to find anything after hours of googling. Please also note that I didn't ever reboot the server after the first grub update that I mentioned in the first paragraph of this post.

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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

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