Linux – How to install grub on multiple disks

You need to install GRUB to the MBR of both drives, and you need to do it in a way that GRUB considers each disk to be the first disk in the system.

GRUB uses its own enumeration for disks, which is abstracted from what the Linux kernel presents. You can change which device it thinks is the first disk (hd0), by using a "device" line in the grub shell, like so:

device (hd0) /dev/sdb

This tells grub that, for all subsequent commands, treat /dev/sdb as the disk hd0. From here you can complete the installation manually:

device (hd0) /dev/sdb
root (hd0,0)
setup (hd0)

This sets up GRUB on the first partition of the disk it considers to be hd0, which you've just set as /dev/sdb.

I do the same for both /dev/sda and /dev/sdb, just to be sure.

Edited to add: I always found the Gentoo Wiki handy, until I did this often enough to commit it to memory.

Personally I prefer grml.org's rescue cds.

First of all the most important is to get a cd that matches your $arch like i386/amd64.

• You boot the cd and assemble the raid/lvms.
• mount only the root fs
• chroot into the root fs
• outside the rootfs stop udevd
• inside the rootfs start udevd
• assemble/scan/mount the missing filesystems inside the chroot (the most important is the usr and boot)
• rebuild the initramfs
  • on debian (i think it is the same on ubuntu) run:
  • update-initramfs -uv -k $yourkernelversion
  • update-grub or update-grub2

If something else is wrong like lvm2 uninstalled by accident or something like that you can reinstall it in the chroot. If you give the exact error message we try to take a look.