Linuxcnc live usb does not work, cd works

Hi I am new to the world of linux:cheer: . I installed ubuntu on an old mac and it works well. Now I want to run linuxcnc on it. I burned the live cd and it works, I install it and the computer starts up again and there is a question mark on the screen. I have installed ubuntu back on it. and try to make a live usb and not work at all. Who can help with a simple explanation so that I can run linuxcnc on my old mac?

The Mac will not run a live USB Stick as it blocks the Kernal to interfear the Hardware
While the DVD shidt do the Job

When I install the DVD I can finish everything through the entire menu. then I restart the computer. and then I get a question mark on the screen. And can then do nothing more. Do you have a solution?

at startup press E
the manuell start menue shows
then go to line
GRUB_CMDLINE_LINUX_DEFAULT="quiet splash lapic"

if there is no lapic enter it and restart with X

When I press E, the computer just continues and opens ubuntu

If I press X, the computer also just starts and opens ubuntu

I hold shift and the computer starts and opens Ubuntu

Should perhaps tell that it is an imac

why dont you just use a Very Cheep 25USD old 2Core PC to get it in a workshop a mac is not the best to get