This thread is to setup pi4 to boot from SD card and load Ubuntu-Mate from USB3 flash drive or SSD. I use the Rasberry Pi Imager program to install Ubuntu 64 bit 18.04.4 onto an SD card. After booting on the Pi4, the Mate desktop can be installed. I inserted a high-speed USB3 drive and created a partition for the OS. Then I copied the OS to the USB using rsync.
What is the next step to direct the boot on the SD card to the OS on the USB?