Mac not showing external flash drive with turnkey linux installed

I have been trying to setup a spare iMac (iMac12,2) that I have as a minecraft server, but whenever I try to make the flash drive installer, the mac doesn't recognize the drive in the boot picker (holding down option at boot)... the drive is bootable because an old macbook (MacBook4,1) that I have is able to boot.. I've tried installing rEFInd but when booting into the drive it just displays a black screen, and when using clover bootloader, it just boots into the GRUB bootloader... any ideas?

Solution 1:

This may be far fetched, but is the USB formatted as GUID partition scheme? The Mac boot program rejects anything other than it with my experiences. And have you checked if the Linux is UEFI compatible? That may be an issue with newer Mac's.