I understand that you want to use your external hard disk as your installation source, in exactly the same way you would use a DVD or USB stick. If this is the case, a simple command can do what you want but warning, it is a very strong command and incorrect use may and will destroy data completely, so use it very carefully:
sudo dd if=iso_file_path of=/dev/sdx bs=4M && sudo sync
where x is your external hard disk letter.
- Make sure the external drive is not mounted when you run this command.
- Make sure you use the correct drive letter otherwise you will wipe all data on whatever drive the letter you use refers to, even if it is your system drive, without any prior notice or verification.
You will not see any progress indicator while the .iso is being copied. Just wait for the prompt to reappear after the copy is complete.
After this, you should be able to boot your system from your external drive by using it as boot media from your BIOS's boot menu.