This feature is inherited from NOOBS, so it should work exactly the same.
Kenjutsu wrote:When I copy all the PINN files to a USB flash drive and boot the Pi, I am informed: "Cannot find the drive with PINN files". Clicking on close gives: "Recovery application crashed. Starting shell sh: can't access tty; job control turned off"
Your USB drive should be formatted as FAT32 and all the PINN files should be stored in partition 1 (SDA1 normally). It seems PINN is booting ok, but not finding the correct drive. Maybe there is some issue there. It should not crash so I will try to investigate.
Kenjutsu wrote:When I copy all the PINN files to an SD card and boot the Pi, I get the option to install to the USB flash drive, which I then proceed to do (Ubuntu Mate 16.04.2 LTS for this example). I am informed: "OS(es) Installed Successfully". I click on OK, and the Pi reboots. Ubuntu loads and I complete the System Configuration. The Pi reboots again, I get the Ubuntu MATE welcome screen. From here, I shut down, and power-off, and remove the SD card. I power on the Pi, but nothing happens
Using this method, you must keep the SD card inserted. The BOOT partition is stored on the SD card, so Ubuntu will not boot without it. Once booted from the SD card, Ubuntu will run the rootfs from the USB drive. This method will work with all RPi models.
If you need to run without the SD card, you must copy PINN to the USB drive, boot into it without any SD card inserted and install the OS from there.