Moved root and swap directories, how can I reconfigure GRUB2?

Question

I'm on Ubuntu 11.10 64bit booting using rEFIt on a 8,3 2011 Macbook Pro.

I recently switched out the hard drive and modified the order of the partitions when duplicating the disk. Everything's working fine, it's just that I get a "Missing operating system" message when trying to boot into Linux. I'm assuming that the problem is that what was once root at /dev/sda5 is now something else.

How can I update GRUB2 to point to the right partitions for my root directory and swap? I know which partitions are which, I just need to reconfigure things. (GRUB is installed at /dev/sda4, not /dev/sda, to allow rEFIt to work.) I already tried using Boot-Repair, but it didn't fix things as it still won't boot. I'm currently running from a LiveCD and I've mounted the drive, looking for the right configuration file.

What do I need to change and where?

Answer 1

You first need to identify your root and boot partitions.

Assuming your root partition is /dev/sda4 , and you do not haev a separate boot partition, we are going to mount your root partition, install grub, and update the grub menu.

# mount your root partition
sudo mount /dev/sda4 /mnt

# Mount your virtual file systems
for i in /dev /dev/pts /proc /sys; do sudo mount -B $i /mnt$i; done

# Install grub
sudo grub-install --boot-directory=/mnt/boot /dev/sda

# chroot , update, and then install grub
sudo chroot /mnt
update-grub
grub-install /dev/sda

To fix your swap, list your partitions

sudo blkid

Edit /mnt/etc/fstab with the correct swap uuid

# graphical
gksu gedit /mnt/etc/fstab

# command line
sudo -e /mnt/etc/fstab

Update your swap UUID

UUID=xxx-yyy-zzz  swap  swap  defaults  0  0

Change "xxx-yyy-zzz to your swap uuid ;)

reboot and all should be working.