How to install encrypted Debian/Mint on external drive and not overwrite internal ubuntu's grub?

I would like to install new encrypted debian or mint on external flash disk while already having windows and ubuntu installed in dual boot on internal drives. For booting I use ubuntu's grub on internal drives and I worry about possibility of overwriting this grub after installation of debian/mint on external disk. How to avoid that ? It would be nice to have option in ubuntu's grub to boot into debian on external drive after pluged in that external drive. Can someone tell me how to achieve that ? Thanks in advance.