In order to safely upgrade from 14.04.5 LTS to 16.04.01 LTS I have decided to proceed as following:
sudo dd if=/dev/sda1 of=/path/to/mounted/home/partition/rootpart.imgMake sure that you substitute /dev/sda1 with your root partition!
sudo badblocks -svn /dev/sda1That would initiate a non-destructive write test. In my case I could continue to image the root partition after running the badblock command.
sudo do-release-upgradeIf you get a complaint about your python installation being corrupted, then use the following solution.
That's it! Your upgrade should now be safely completed. If anything goes wrong during the upgrade, or the upgrade leaves your system in a bad state, you can use the image file to restore your 14.04.5 LTS installation.
To do that, boot off a USB stick with the 16.04, mount the /home partition and re-image the root partition from the file using dd:
sudo dd if=/path/to/mounted/home/partition/rootpart.img of=/dev/sda1Make sure that you substitute /dev/sda1 with your root partition!