With a bootable Ubuntu USB stick, you can:
Creating a bootable USB stick is very simple, especially if you’re going to use the USB stick with a generic Windows or Linux PC. We’re going to cover the process in the next few steps.
Boot Mac from USB. Follow the below process to boot your Mac from bootable USB: Insert the bootable USB installer drive and go to “Apple Menu Restart”. When your Mac restarts, press and hold the Option key till “Startup Manager” opens. The Startup Manager scans and lists the drives that are connected and bootable. Make sure the destination disk is the USB thumb drive you've inserted. It can take about 20 minutes to format and set up the USB installer on the Mac.
There are a few additional considerations when booting the USB stick on Apple hardware. This is because Apple’s ‘Startup Manager’, summoned by holding the Option/alt (⌥) key when booting, won’t detect the USB stick without a specific partition table and layout. We’ll cover this in a later step.