I rather stupidly bought some low quality SD cards and found after about a week my PI would fail. I had a few thumb drives around so I used the SD card only to boot up using the instructions from this link https://www.stewright.me/2013/05/instal ... ash-drive/
All went swimmingly and I could change my system by swapping out a thumb drive, easier to keep track of and label than a card. Problems came when I used apt upgrade which upgrades the kernel. The sdcard and the thumb drive I ran the update on are both upgraded (I think) but plugging in a non-upgraded thumb drive and it won't boot.
So using several thumb drives does not seem like such a good idea. Can I boot only from the thumb drive?