100% on the external drive. Every few days I rsync it with an identical one. So if one crashes I can immediately plug in the spare and have an identical system. The code I write is of course spread over several git repositories. So it is more up-to-date than 'every few days'. USB3 drives... so I am quite happy with the speed. And if my computer crashes, I don't have to open it, to get the internal disk... Of course, I don't throw it away, but I don't have to get it immediately. Nothing important on it.... only Windows and a few games.