Kippt från How-to-geek
https://www.howtogeek.com/213396/how...e-on-your-mac/
Apple’s made it difficult to boot non-Mac OS X operating systems off of USB drives. While you can connect an external CD/DVD drive to your Mac and boot from standard Linux live CDs and USBs, simply connecting a Linux live USB drive created by standard tools like Universal USB Installer and uNetbootin to a Mac won’t work.
There are several ways around this. For example, Ubuntu offers some painstaking instructions that involve converting the USB drive’s file system and making its partitions bootable, but some people report these instructions won’t work for them. There’s a reason Ubuntu recommends just burning a disc.
rEFInd should allow you to boot those USB drives if you install it on your Mac. But you don’t have to install this alternative UEFI boot manager on your Mac. The solution below should allow you to create Linux live USB drives that will boot on modern Macs without any additional fiddling or anything extra — insert, reboot, and go.