I recently created a Live TAILS USB using the recommended Universal USB Installer (on Windows). However, when I use the USB to start up on my MacBook Air (macOS Sierra), it does not boot. In the startup disk chooser, it shows the drive 'EFI Boot' mounted, but the computer fails to start on this disk. I tried it on another desktop, a Mac Mini, and it works. When the USB is plugged in, two drives are shown as mounted, one 'Windows' and one 'EFI Boot'. 'Windows' boots TAILS fine. 'EFI Boot' does not work. The Windows drive is simply not recognized on the MacBook. I am considering trying to create a final TAILS and trying to boot my Mac from there, but I would like to avoid this if possible. The Mac Mini is running an earlier version of Mac OS X (El Capitan?), if that helps.

"Windows" just means go into the EFI BIOS emulator, while EFI means boot natively in EFI mode. I'm not familiar with that particular software, but you can snoop around the disk. There probably is a "/boot" directory or something and you can find the main EFI boot file and then run the program "bless" on it with that filename and use the --setBoot option. That should "bless" it as a startup disk for Macs. Read carefully the manpage for "bless".

I have to say though, it's a bit like voodoo art as to how to use "bless" properly to get a drive to boot on Macs if it's not an Apple OS. You can try many different variations of it.

Good luck!