Support for occasionally plugged hard drive for steam-like application?

Solution 1:

Steam, GoG, Origin, Epic, Blizzard Battle.net, and UPlay all support using an external drive.

Symbolic links (Wikipedia) would work no matter if the launcher supports external drives or not, but may be problematic if the external drive isn't plugged in when you attempt to play a game as they will "not exist" and it may start installing the game or just throw an error. Also important to distinguish between "symbolic link" and "junction" on Windows as junctions are handled differently sometimes, especially by non-standard programs.

The "Symbolic link" Wiki entry above has a nice table of differences between symlinks, junctions, and hard links. I excluded hard links from this because they are not desirable as they can't cross filesystem boundaries and don't even support folders.

So, in short, symlinking folders is an option if you use a launcher that doesn't support external drives. Just make sure to symlink the folder of the game rather than the launcher's folder, e.g. for Steam you would move the installed app data from Steam/steamapps/common/appname to the external and symlink from there to appname on the external, like ln -s /mnt/external/games/appname .../Steam/steamapps/common/appname.