Hello Cowboy,
I read that you may need to also disable fastboot in BIOS. And while you are at it, you might want to check to see if you are running the latest BIOS. You can visit Dell here to get the latest BIOS firmware installed, please note you will have to check all of your BIOS settings after performing the update.
Lastly, if you take a look here on Dell’s site, you will find someone walking you through every stem of dual-booting Ubuntu. Come back and let us know how it goes!