grub-install Cannot find a GRUB drive for /dev/sdXY

[Musbur <musbur@posteo.org>]

Hello,

I somehow managed to screw up my /boot partition. Now I'm trying to 
re-install it by booting from a Debian installer stick. After network 
configuration, I do this:

1) Mount the root partition at /
2) # for s in dev sys proc; do mount --bind /$s /mnt/$s; done
3) # mount --bind /etc/resolv.conf /mnt/etc/resolv.conf
4) # chroot /mnt
5) mount /dev/sda2 /boot

Now I'm in my "full" Debian system. Next:
# grub-install /dev/sda2
# apt install [linux kernel]
# update-grub
Error: Cannot find GRUB drive for /dev/sda2

I tried both the grub-pc as well as the grub-efi versions. I don't 
really know what the difference is and how it works, but with EFI I get 
a similar error (something like EFI directory not found)

BTW /dev/sda is an external SSD connected via USB3. I've been using this 
setup for years to boot my employer's company Windows laptop as my 
private Linux machine (not touching the internal SSD at all). It stopped 
working from one day to the next, no idea why.

I've searched the Internet up and down for the error message, but I 
couldn't find anything useful.