This is not strictly a HASS issue, but it led me in frustrating circles for days when trying to install HomeAssistant on a USB drive for the Raspberry Pi 3+
It turns out that even with the “boot from USB” flag turned on, the Pi cannot boot from some newer USB drives! This is especially true for USB3.0 drives. So if you have trouble getting your Hasbian or Hass.io system to boot from a thumbdrive, try another brand.
I had a SanDisk USB3.0 32GB drive and it will not boot. I swapped to an older Patriot USB2.0 16GB drive and it works perfectly.
Hope this helps save some “hair pulling” frustration for others.