Restore grub

My server was installed a long time ago and it seems that the partitions were created in a way that grub does not like. And when grub does not like something you have good chances of having a computer not booting.

Since I have the problem from time to time, let's write a reminder on how to fix this.

Sometime there are grub updates, and on the next reboot, well, nothing happens. Hopefully, my hosting provider provides an option to boot my server on a Ubuntu live CD (my server is running Debian but that's fine).

When Ubuntu has finished booting, I can ssh on it and list my hard drive partitions:

# fdisk -l /dev/sda
(…)
 Device Boot      Start         End      Blocks   Id  System
/dev/sda1               1          12       96389+  83  Linux
/dev/sda2              13       19324   155123640   83  Linux
/dev/sda3           19325       19454     1044225   83  Linux

Here sda1 is my boot partition, sda2 is my root partition and sda3 is swap. Let's mount that:

# mount /dev/sda2 /mnt
# mount /dev/sda1 /mnt/boot

Now we have to change the root directory from the live CD to my server's disk so we can simulate being running on my server's disk:

# chroot /mnt

And now we can reinstall grub:

# grub-install /dev/sda

In my case I have to give the --force option to install grub. That's all, I can now restart my Debian server.