Perhaps I need to take a course in how to use search engines (or perhaps search engines need to take a course on how to search for what I ask for, not what is convenient to find.) Off my soap box now
I have several Pi 3B+ models running Ubuntu Mate 18.04 from MicroSD cards. I wish to boot one from a USB flash drive. I have found how to boot a Pi3B+ to Raspbian from a USB drive (which I do NOT want to do) and how to tweak Ubuntu to run on the Pi 3B+ (which I already know how to do.)
Would someone please tell me if a Pi 3B+ will run Ubuntu from a USB drive? and if so, what tweaks to the OS image are needed? I cloned the working Ubuntu image to a Samsung Fit flash drive and installed it in place of the MicroSD card in a Pi 3B+. It "boots" to the 4 raspberries screen and the green led flashes. That is as far as it goes.