Problem – MacPro 1,1 is 32-bit EFI while having 64-bit architecture. There are other ways to boot that bypass EFI mode, but – booting in EFI mode on MacPro 1,1 allows for 2 more SATA ports (located on motherboard).
To run a live Ubuntu 18.04.1 USB on MacPro 1,1, one needs to add a 32-bit EFI boot loader, here is how to do it: