USB boot is available on the Raspberry Pi 3B, 3B+, 3A+ and Raspberry Pi 2B v1.2 models only.
This tutorial explains how to boot your Raspberry Pi from a USB mass storage device such as a flash drive or USB hard disk. Be warned that this feature is experimental and does not work with all USB mass storage devices. See this blog post from Gordon Hollingworth for an explanation of why some USB mass storage devices don’t work, as well as for some background information.
Source: How to boot from a USB mass storage device on a Raspberry Pi – Raspberry Pi Documentation
Related article: How to Boot Up Raspberry Pi 3 from External Hard Disk – Make Tech Easier
