Booting from an external USB drive is definitely possible and very easy to do. All computers offer a kind of 'one time boot menu' for both legacy and UEFI boot modes.

Mounting and unmounting drives using both the terminal and the GUI is an essential Linux skill. In this article, we show how to do this and how to work with ISO image files.

Thanks to open-source gem UNetbootin, you can quickly and easily transform a flash drive into the ultimate rescue tool.

Here's how to create your own disaster-recovery boot drive with Linux, and how to force Windows to shut down installing updates.

This appears to be something Apple has changed recently to only allow Macs without installed internal optical drives to boot from non-apple external disks, whereas they worked happily before.