Unable to boot an iMac running OSX 10.6.8

I have created 2 ‘bootable’ USB drives using your galena-Etcher app, one is a 4 Gb stick, the other a 8 Gb stick, on which I have flashed Linux Mint 21.2 OS.

I have “Live Session” tested both of these USB drives successfully on two iMacs, one running OSX 10.11.6, the other running 10.13.6. Additionally, I have successfully tested the drives on both an Acer desktop running Win 8, and an HP laptop running WIN 7.

Therefore I believe both USB drives are accurate and usable.

My problem / question involves 3 other Macs that are all running OSX 10.6.8. All three of these iMacs fail to show the USB drive as a ‘bootable’ option, recognizing only the main HHD as a ‘bootable’ source.

Surely there must be some way these machines can be made to boot from the USB drives that work well on several other machines, but I have not found a way.

I am not a developer or programmer so I’m not very technically inclined, but I do think there is likely a solution available that I have not yet found. Would I be more likely to be successful on these machines if I tried using an optical drive?

Thanks for any suggestions that may resolve this situation.