I am not a huge fan of the built in Win8 backup program but still using it for now.
What I want and I know I seen it somewhere (even wiling to get the paid version) of a program that you install, and configure what folders/files you want synced with your external HDD. So each time you connect it whatever changes are made to the files are re-copied to the external much like a differential backup.
Things to know:
I am NOT interested in Online Backup
I am not interested in taking an image of my PC
I only want specific directories backed-up
I am fairly certain I seen something like this with high ratings but can't remember the name of it for the life of me.
Oh, and most importantly, I want them to be flat files