Hi,
I used to have XP11 on a laptop and ortho sceneries on an external hard drive before I built my dedicated PC for X-Plane.
It worked quite well. Just create a folder on your external HDD and name it Custom Scenery. Place your ortho tiles into this folder. Inside this folder create shortcuts for all your tiles.
Cut out these shortcuts and place them into the Custom Scenery folder of your XP11 on your laptop. Then delete the scenery_packs.ini file there. Next time you start XP11 a new one will be created with everything in the right order. ( provided your addon sceneries etc. were correct prior)
As for the type of external HDD you use: any good HDD with a USB connection will do. I used WD Elements and I was happy with them.
Hope this helps,
Werner