How to set up a Raspberry Pi 4B to boot Ubuntu Mate ARM64 from USB
Guide Overview
The Purpose of this Guide is to show you how to set up a Raspberry Pi 4B to boot from a USB SSD using Ubuntu Mate ARM64.
Raspberry Pi's have come a long way. The latest model 4B is a nice little micro computer, that rivals some low end laptops, and possibly some mid range ones, as well.
The Pi-4B comes with 2, 4, or 8GB of RAM. If you are looking at building a desktop Pi, then I recommend the 8GB board. Remember, they are SoC systems, so you can't add extra memory to them afterwards.
Tools Needed
Any PC, Linux, Windows, or Mac, with the Raspi Imager installed.
A functioning Raspberry Pi-4B with a recommended minimum 4GB RAM.
A minimum 8GB Micro SD card, 32GB Recommended.
Either a micro SD port on the PC, or a Micro SD card adapter.
An External USB Drive, SSD recommended.
A copy of the Ubuntu Mate ARM64 image, 20.10 or later.
NOTE: If the external USB drive draws more than 600mA, a separate power source is required for the drive!
For the purpose of this guide, I am using an 8GB Pi. It is attached to the back of a 24" Philips monitor, using a Vesa mounted enclosure. It has a 64GB Sandisk Ultra Micro SD Card, and a 512GB Kingston PCIe NVMe M2 SSD mounted in a Sabrent USB 3 enclosure.
Instructions
Step 
- Prepare the Micro SD card
If you have not already done so, install the Raspi Imager program on you PC or Mac
Insert the Micro SD card into your PC or Mac, and then start the Raspi Imager.
Under "Operating System" select the default Raspberry Pi OS (32 bit.)
Under "Storage" select your Micro SD card.
Write the image to your Micro SD card.
The Raspi Imager will download and install Pi OS (32 bit.)
Step 
- Boot the Pi 4B and run all updates.
When the Pi OS boots for the first time, it will expand and install it's self on the Micro SD card.
Go through the set up for the Pi OS.
Once the Pi OS is set up, reboot the Pi 4B to show that default settings are working properly.
Open a terminal (<ctl>+<alt>+<t>) and run the following:
sudo apt update
sudo apt upgrade
sudo rpi-eeprom-update
sudo apt install rpi-imager
This will ensure that all software and firmware are up to date.
Step 
- Connect and Install Ubuntu Mate on USB Drive.
Connect your USB drive to Pi 4B. SSD drives should connect to the USB 3 ports, to take advantage of higher speeds.
Open the web browser and download the Ubuntu Mate ARM64 image.
NOTE: Ubuntu MATE 20.10 and newer support USB booting, but Ubuntu MATE 20.04 does not. Ensure that you download Mate 20.10 or later.
Start the rpi-imager from the terminal, or Imager from the desktop menu.
WARNING! All existing files on the USB drive will be erased! A new partition table and Ubuntu Mate will overwrite the drive!
Under "Operating System" select Custom, and navigate to the Ubuntu Mate ARM64 image.
Under "Storage" select your USB drive.
Write the image to your USB drive.
Step 
- Set Pi 4B to boot from USB
In the terminal, start the raspi-config utility.
sudo raspi-config
Select "Advanced Options" from the menu.
Select "Boot Order" from the Advanced Options Menu.
Select "USB Boot" and then tab to <Ok> and hit enter.
A note will pop up telling you that USB is the default boot device. Hit enter to clear the pop up.
Tab to <Finish> on the menu and hit enter to exit the menu.
At this point you have the option to return to the Pi OS, or reboot the Pi.
When it boots it will bypass the Micro SD card, and go to the USB drive.
Run through the setup for Ubuntu Mate, after which it will become your default OS on the Pi.
POST INSTALL NOTES:
- You have the option to remove the Micro SD card, if you wish. Set it aside, and mark it for use on the Pi. It will be your recovery method, if your system fails to boot from the USB drive. You will also need it if you wish to change the settings in the Pi4 eeprom.
- To boot back into Pi OS, ensure the Micro SD card is installed, and unplug the USB drive. The Pi will fall back to the SD card if it fails to find a USB boot device.
- After Ubuntu Mate 20.10 is installed, it will offer the option to ugrade to 21.04. I suggest declining the option, and working with the system for a few days to ensure that it is functional and stable. You can upgrade at a later date, by using the terminal command "do-release-upgrade".
- Since upgrading to 21.04 I am getting a "System Error" on login. After looking at the /var/crash log, I think it's related to CUPS, but I am not sure. I will looking into it later. For now, after clearing the error report, the system appears to function normally.
- There are known issues with the Logitech Unifying Receiver not working properly with the Pi4. It fails to detect the receiver, and you are left with no keyboard or mouse. The Logitech MK270 KBM combo would not work.
- Surprise! The cheap $20 house brand from Staples does. So does a few other non Logitech KBM's I tried. I suspect the error is with the 2.4GHz band, used by the receiver. Because it is also used by Bluetooth and WiFi, it may be interference. I am investigating further.
Enjoy you piece of the Pi!
Cheers!
Naught 
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