Kali Full install USB not recognized by Mac

I have been trying for 4 days to do a full kali OS install on a usb drive (not a live usb, FULL). I can not find anyone facing a similar situation anywhere I look. I have created a live usb (via dd) to do the install and the live usb works perfectly fine on the Mac. Once I install kali onto the desired usb, however, the Mac bootloader refuses to recognize it or even show it as an option to boot from. Any suggestions or similar situations?

Kali version: 2018.4 amd64
Mac info: iMac (Late 2013), OS X Mojave

Not a Mac user, but usually I have to disable my onboard hard drive as a boot source in order to boot from a USB or DVD. You might find that sidesteps the issue. Obviously it's not ideal.

As to why it happens... I think I have a very outdated BIOS with a bug in it. If there's a hard drive connected into the onboard socket, it just insists that has the highest boot priority. Makes multi-boot very fiddly to setup.