I've installed Ubuntu 12.04 to an external Hard Drive, planning on having Dual Boot on my laptop - which runs Windows 10. I have successfully configured the Dual Boot to have Windows as default, ran boot repair and everything, but whenever I don't have my external HD plugged in, I get the GRUB “MINIMAL BASH LIKE LINE EDITING IS SUPPORTED” black screen. I would like to be able to use my computer even without the external HD. One more detail: I selected the external HD as the device for boot loader installation, which might have caused the problem.
I agree with you as to the cause, it looks like you've installed the bootloader on your external HDD, so when you turn the PC on its looking for that GRUB install to work out what to do. Naturally, if it's not plugged in the PC doesn't know what boot options to give you.
Seeing as your installation is still working fine, I'd suggest plugging the external back in, booting into Ubuntu and using Boot-Repair to put the bootloader back on your internal drive. Have a read of the following for info on how to use it and what it's for: