I've been searching around for a while and have not really found an answer to this question other than install rEFInd…which I want to reserve as a last resort. If that's the only answer, then it's the only answer. I just want to be sure.
I have had no issues creating a bootable USB drive with the 17.04 ISO, and my MacBook Pro has no issues booting from that USB drive to run 17.04 live.
What I want to do is install 17.04 to another 64 GB USB drive, be able to boot off of that USB, and run 17.04 like it is installed on my hard drive.
To prepare, I used Gpartd in the live environment to create a GUID partition table on the 64 GB USB stick, created a EFI system partition at the beginning of the drive (FAT32 marked as ESP and Boot), created a swap partition (primary linux-swap), and created the main partition (primary ext4).
I started the 17.04 installer, instructed it to put GRUB on the ESP, and install the root file system to the ext4 partition. Everything ran fine, but, of course, the MacBook Pro's boot manager does not recognize it. It still recognizes the install USB stick just fine.
Any help would be appreciated. Like I said, I would rather avoid rEFInd just because I like to make as few changes to the system as possible. But, I'm cool with that being the only answer.