I am in the process of migrating from Ubuntu to Debian, and for logistical and health reasons the two laptops (Samsung and Asus) are miles apart and are not to be moved.
On the Samsung, I have Ubuntu 18.04 and Debian 10 co-existing on the hard drive.
Then I did a full install of Debian 10 on the Flash Drive and went through two steps:
1) Installing GRUB on the Samsung prevented me from accessing the Buster on the Flash Drive even after update-grub.
2) So, I went back, installed GRUB on the Flash Drive, update-grub, and lo and behold, I can access the three systems.
The drawbacks which I see (there may be more) are: a restart of either the Ubuntu or the Buster residing in the laptop does not work, only a shutdown will do; and that also means that I have to have the Flash Drive inserted at all times.
The situation is not all that bad from a security point of view: the Flash Drive is acting as a soft and hard key to grant access to the Samsung laptop. Not all that bad but not all that elegant either.
The Asus already has the Debian 10 freshly installed. To complete the strategy, I should insert the Flash Drive on the Asus and have a situation exactly parallel to the one working on the Samsung laptop. But before doing it, I would be grateful to hear your comments, no matter how unflattering

, on how to go about this.
Thank you for your help.