We are just not communicating. I don't blame you, it is me not being able to express myself effectively. Let me try again.
You seem to want to change the boot with some key combination/BIOS setting. Don't
. You do all the boot selection from the Grub menu, not by changing the boot drive. You set the BIOS so it boots from the internal drive, the one that has Windows on it. You do this when the external drive is plugged in. To do this you boot with the external drive plugged in. You should see the internal drive as sda1, and it should have Windows on it. Once you have this set in your BIOS you leave it alone. You do not
change this to usb boot, sda2 boot or any other option available in BIOS to boot OSs.
You make the install of Mint with the /boot partition on sda1 and the / partition on sda2 and let the installer put the grub kernel in the boot sector of sda, hd0.
Without changing anything, the external drive still plugged in, when you boot you should be able to boot into Windows or Mint from the grub boot screen. When you boot without the external drive plugged in you should be able to boot Windows but not Linux from the grub boot screen. The reason being that the Mint install is on the external drive which is not plugged in.
At no time should you try to change the boot drive from the BIOS. Always use the grub boot screen, with the BIOS set to boot the internal, sda drive. That should never change regardless of what OS you are trying to boot.
I hope this is making some sense to you.