Just place them all there. OTOH, it makes litle sense, as they won't be accessible when the external drive isn't plugged in.
git repositories are just plain directories full of normal files, that can be backed up, copied and moved around at will. What I do with my really important stuff is to have an external repository (on another machine I have set up a bare
git repository for the project) and now and then I push any changes over (OK, around once a day). Others that I share are also on a local git server will all bells and whistles. Same idea as before, changes are regularly uploaded,