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GRUB trouble, Help needed

Hi.

I have been building a maintenance system on an USB stick, and botched
it. I've used the Ubuntu installer and the MBR got written to the wrong
disk, namely the first hard disk (an SSD in my case) instead of the USB
stick.

I've done it again, doing it right, and can now boot from the new USB
stick. But I can't boot my main system any longer, which had been
started from /dev/sda.

I can access my encrypted root partition from the maintenance USB
stick, and mounted it to /mnt/root. Then I've tried this:

mount --rbind /dev /mnt/root/dev
mount --rbind /proc /mnt/root/proc
mount --rbind /sys /mnt/root/sys
chroot /mnt/root
update-initramfs -u
grub-install /dev/sda
update-grub

This is by Colin Watson in ubuntu-users at Mon, 3 Sep 2018. He said it
goes "roughly" like this.

What am I missing?

Greetings,
Volker