I searched, but couldn't find the answer for my situation. In fact my problem considers two questions.
Here's the thing; I have Linux Mint 18 installed on my computer, and I tried to install an 'independent' Ubuntu Studio version on a 32 GB USB drive. (So not just as a "live" version, but as a complete OS on its own)
The intention was to have a situation like: If no USB drive is plugged, Linux Mint will boot, if the USB drive is plugged, either Ubuntu studio boots, or I'll get a bootloader to choose one of the two.
In order to (attempt to) achieve this I created a Linux Studio live DVD, I started this on my Mint system, I chose the option: 'Install Ubuntu Studio', I chose the USB drive to install it to (first I got an error, but after choosing '/' as mountpoint on the USB drive it continued) Then I was asked where to install grub too. I prefered to have no bootloader at all, but if it must, then it should only be there if the USB drive was plugged. So I chose to install Grub to the USB drive. The install process continued without any problem.
However the end situation is that I can only boot my Linux Mint now when the USB drive is plugged. Without the USB drive plugged I get some Grub command prompt. You can guess that it is quite irritating to be obliged to plug the USB drive everytime I want to boot the default OS, Linux Mint.
So ome thing is sure; I made one or more mistakes during installation. Which brings me to my two questions; 1) How can I restore/remove Grub in such a way that the default OS will boot without the need of a USb drive 2) Is it even possible (and how) to install a second Ubuntu OS on a USB drive in a way that it acts like a complete OS (So settings, updates, additional installed programs and data will completely stay on the USb drive. And 2b) Can I keep them apart without a bootmanager (So only via the efi settings)?