Ubuntu Mate 18.04.02 USB Boot on Raspberry Pi 3B+


I have successfully downloaded and installed Mate 18.04.02 on to an SD Card and this boots great and works fine on my RPi3B+. I then tried doing the same thing using a USB stick and it just does not work if I have no SD card plugged in.

I have read everything I can find but still can't make it work. Help

I used etcher on my mac to burn both the SD card and the USB stick if that helps. All the info I can find claims this should work on RPi3B+