If you own a Raspberry Pi, you can start several operating systems from a single boot medium. All you need is a micro SD card with sufficient room (32GB or more) and a tool called PINN. This stands ...
The firmware included with the Raspberry Pi 2 Model B version 1.2 introduced the possibility of booting from a USB device: before that, it was only possible to boot from an SD or microSD card. But ...
Raspberry Pi and other SBCs can boot an operating system from SD cards and USB devices. I own multiple SD cards and am a fan of swapping them to instantly switch to a different OS, such as RetroPie, ...