Since this isn't necessarily a tutorial, at least not in the conventional way, I am sticking this here in the general section of Bleeping Computer's Linux forum (and not naming it "how to"). This is more of a cronical of how I got dual booting off a external SSD on the type C port to work....
This journey started off with me wanting to dual boot my Dell XPS 13 (9350), but since I only had a 256GB internal SSD I didn’t want to break it up into separate partitions and lose space (especially since Windows is a storage hog). The great thing about the new XPS 13 is that it has the USB type C port on it (or thunderbolt 3). Its way faster than the USB 3.0 and can do a multiude of different things, you can get a dock from Dell that runs off the port and has everything a conventional dock might have.
I had started off wanting to dual boot with Kali Linux (no I am not going to give you hacker wannabe script kiddies help with Kali – not that I know a ton either) but quickly discovered that Kali does not like this XPS 13. I had wanted a nice portable Kali install since I am actually in the security industry and have a need to learn more about doing penetration testing, but alas it wasn’t meant to be. So back to my typical Linux install, Ubuntu (in this case the new 16.04).
So to cut the story short a bit and not to drag it out, I had messed around with how to boot off this external SSD I had gotten, but wasn't having a ton of luck (most of it due to Kali not liking the XPS and also not recognizing the type C port). There isn't a ton out there on booting off the type C and the XPS so I kind of had to muck around. One thing I knew, was that secure boot needed to be turned off. Luckily Dell kept their standard bios, even on the newer PCs. All you have to do is make sure fast boot is turned off (W10 or W8/8.1) and hit "F2" while the PC is starting. The option to turn off secure boot is in there.
Once that was done I booted to Ubuntu from a flash drive (also had to enable legacy ROMS in the bios to do this) and was able to successfully install it to the external SSD. As a note, when installing to an external make sure you isntall the boot loader onto the same drive, that way you don't screw around with your main Windows (or whatever you are running) install. Now just to try and get this thing booted....
Restarted and went into the boot selector (hit F12 on bootup), well shoot, there is no Ubuntu listed even though I know I installed it. Long story short, and some more poking around in the BIOS (and a couple hints from off the internet) I discovered that there is an option to let you boot off the type C port, "Enable Thunderbolt boot support". Once that is checked you are good to go and can boot off the USB type C port!
So what is the TL;DR of this you might ask (and TL;DR is 'too long; didn't read' for non reddit users)? Quite simply this:
Step 1: Disable fast boot
Step 2: Disable Secure boot
Step 3: Enable Legacy option ROMS (so you can boot off the USB)
Step 4: Enable Thunderbolt boot support
So after that ridiculous wall of text, that only half of you read, that is how you boot off an external SSD on the new type C port.
Oh, and for the few minutes I have ran Ubuntu off it, there is literally no performance loss that I could tell. Most than likely due to running off an SSD and the new Thunderbolt port being ridiculously fast.