I agree with C.S.Cameron,
It is a good idea to do full installs to a multi boot USB drive. If you disconnect or unplug the internal drive before you start installing, the procedure will be like it would be, when you create a dual boot or multi boot system in an internal drive. (After installation you can plug the intermal drive back into the computer.)
See the following links and links from them,
help.ubuntu.com/community/Installation/FromUSBStick#Notes_about_speed
Try Ubuntu (Kubuntu, Lubuntu, Xubuntu, ...) before installing it
help.ubuntu.com/community/Installation/UEFI-and-BIOS
askubuntu.com/questions/786986/boot-ubuntu-from-external-drive
Booting USB drives with grub2 and iso files 'grub-n-iso'
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