I would like to install Ubuntu to an external hard drive without touching anything on the already existing hard drive and OS installations. By doing this I am able to boot from the USB by selecting which drive to boot on startup.
I have always done this by selecting to do a custom installations on the Ubuntu installation wizard, setting the partitions myself and select the USB drive at the drop down list of where to install the boot partition.
I have run into a problem however now with a new PC and I believe it might have to do with Uefi as I have never had one before.
I tried installing Ubuntu 16.10 and even though I have selected for the boot loader to be installed on the USB drive, Ubuntu will instead move/override the existing grub partition to the USB and add both the new OS (on the USB drive) and the old one (on the internal drive) to that. This makes the internal Ubuntu installation unbootable when the USB drive is not connected since Grub is inside.
I also restored the internal OS to its initial state however when I try to select which drive to boot to, the USB drive which now contains the grub with both OSes is not in the list.
Am I missing something? How can I install Ubuntu to a hard drive which I can plug and use without messing anything whatsoever in the internal disk like I used to.