Running an Ubuntu 16.04.x LTS system that boots in Legacy HDD mode on SATA0 a 500GB DIsk according to the following test:
Last login: Fri Mar 2 09:02:23 2018
root@saturn:~# [ -d /sys/firmware/efi ] && echo "EFI boot on HDD" || echo "Legacy boot on HDD"
Legacy boot on HDD
I have 2 external USB Hard disks(1TB and 5TB) as well and recently I'm running into a problem that if the USB Drives are powered on/connected the system will not boot. It gives an error that the UUID of the Internal HDD cannot be located. I seem to recall something to fix this in the boot setup but I can't recall or locate details. If I unplug the external drives it works and boots normally.