You can only install a generic image into the external storage. If you install other images to the external storage, then the image will not bootup correctly.
There are several ways to install the OS into the external storage, e.g.
USB storage, etc.
Etcher has a user-friendly GUI for beginners, and it is also compatible with Mac, Windows and Linux. Simply select an image and it will automatically identify your flash drive and burn the image.
Alternatively, utilise the
dd command (for example from within the Ubuntu Terminal):
$ sudo dd if=/path/to/image of=/dev/sdX bs=1M && sync
/dev/sdX with correct node.