I created a second partition on my main drove to install steam games, that way they persist in case I want to reinstall Linux or access from a portable windows install, but wanted it to mount on boot.

In partition manager, for some dumb reason, I presumed that flagging it as 'boot' would mean just that.

Obviously this was wrong, and now I can't unflag the drive for some reason. It says it applies the changes but just leaves it flagged.