My wife's old computer is an Atom Mini ITX motherboard stuffed into a Chinese cooky tin. She runs Ubuntu from a USB3 external drive, (plugged into the USB2 ports).
When we travel, she plugs the bootable external drive into whatever computer is available and she is happy.
For her birthday I bought her a new Gigabyte Brix with plenty of USB3 ports and an i7 processor.
I want to do a setup that uses the Brix SSD for "/" and continues to use the home directory on the bootable external drive.
I do not want to have to sync home folders before and after traveling, just unplug the external drive and go, with everything already up to date.
What is the safest way to make such a setup?