Install Multiple Ubuntu Versions on the Same Computer

 You can have multiple versions of Ubuntu on the same computer using grub to switch between them.

1. Install the first OS to the first disk
2. Install the second OS to the second disk

If using multiple disks for different versions, after installing the second OS use grub to select the default using the first OS/disk. e.g. to select the second OS/disk as default boot:

1. Log into the first OS/disk
2. Edit grub: sudo vi /etc/default/grub
3. Edit timeout to sufficient to see the menu e.g. 5
4. Save, exit and run: update-grub
5. Reboot and note the number of the second install in the list (index 1), but load the first
6. Edit grub: sudo vi /etc/default/grub
7. Edit default option