How do I update grub in a secondary disk?

lucmove · 09-03-2020, 12:22 PM

Let's say I have everything running hunky-dory on /dev/sda.

Then Grub gets damaged.

I boot up from another disk and connect the damaged one over USB. What used to be sda is now sdb.

I run:

# update-grub2
# grub-install /dev/sdb

The problem is, the two disks have differences between them and grub is going to reinstall itself on sdb with information scanned from sda. When I put the old disk back in its place and boot from it, Grub is going to complain that it couldn't find the root partition etc.

How do I tell update-grub2 ("a stub for grub-mkconfig" according to the man page) to scan sdb so that grub-install /dev/sdb updates sdb accordingly?

One way to do that is booting up from a Grub rescue CD, but that is very inconvenient when I am preparing a secondary disk to be ready to boot in case something evil happens to the primary one. I want to do the job right from the same command line, without having to reboot from a CD.

How do I do that?

TIA

michaelk · 09-03-2020, 02:21 PM

Basically, use chroot when running from the second disk

Code:

 
Mount the old root partition to /somewhere 

Bind mount the virtual files system i.e /proc, /dev , /dev/pts /sys to /somewhere/...

chroot to /somewhere

grub-install

update-grub2

exit (to switch back to the second disk)

unmount everything above.

There are howtos that go into more detail.