This will cover how to write an Ubuntu ISO image to a USB drive on macOS.
Note: this process should work for all macOS 10.x verisons.
Ubuntu ISO image file: https://www.ubuntu.com/download/desktop
Utility to write to USB: UNetbootin https://unetbootin.github.io
Warning: writing the images to USB drives in this guide will remove any data on the target drive.
1 Format USB Drive
To make sure you can see your USB drive in the UNetbootin utility, we recommend a format of the USB to MS-DOS (FAT). You can use spotlight to find and run Disk Utility.
Select the USB drive and click on Erase.
Format to MS-DOS (FAT).
2 Download and Launch
After downloading the Unetbootin utility: https://unetbootin.github.io
Launch UNetbootin, this will require to accept a security prompt for running programs outside of the app store and then a password prompt to run.
3 Select Distro
Select the required distro to download and write to USB. I've selected 'Ubuntu 18.04Live
4 Launch UNetbootin
Now you should be able to re-run the 'UNetbootin' utility and see the USB drive listed in the 'Drive' field.
5 Write ISO to USB
Now click on 'OK' to proceed with a download and write to the USB drive.
Warning: This will overwrite the selected device, so the advice is to make sure this is the only USB drive plugged in.
6 Download, write, complete
The next steps are automated. This will download and write to the USB drive.
Note: This may take time for the download.
After this process is complete, you will have a bootable recovery USB drive.