Whenever I want to test a new release of Ubuntu I find myself cloning the root partition so I can safely upgrade and still keep my old install.
However, I usually forget some step and end up needing to debug grub boot. So this is a summary of what needs to be done in order to clone the partition and make it work. (Almost all of the below need to be run as root of course.)
First you need the file systems in question to be unmounted, so likely you will want to boot from a USB stick or some third partition.
1. Cloning with
The simplest way to clone the whole FS is to just
dd the partition into
another equally sized partition (I keep a few root-sized ones in reserve).
That is, something like:
dd if=/dev/sdd1 of=/dev/sdd2 bs=$((1024*1024*16))
That copies the contents of
/dev/sdd2 with a large enough
block size that you get most out of SSDs.
2. Changing the UUID
Since there are now two partitions with the same UUID, you need to rename one. The command depends on the file system in question, but for ext4 it is:
tune2fs -U random /dev/sdd2
Now you can check that the UUID changed and record the values:
3. Updating the UUID in
The next step is to update the fstab in the changed filesystem. Mount the FS somewhere and edit the root UUID manually or just use sed:
sed -ie 's/<old_uuid>/<new_uuid/g' ./etc/fstab
4. Updating grub
Next you can boot into the normal system (which for me is usually the clone's
original copy, i.e.
/dev/sdd1 above), chroot into the clone FS (
and update grub from there. Go into the root of the clone and run:
mount --bind /dev ./dev mount --bind /proc ./proc mount --bind /sys ./sys chroot . update-grub
update-grub outside the chroot, in whichever install owns
the default drive's grub.