Clearing an external hard dive

I have a seagate 80 gb external hardrive. I use it to back up my laptop that only has 40 gbs. i have been doing this a few years but now there isn't any more space on the seagate. I first though it was doubling everything up on the external harddrive after every backup, but after running double killer and only removing a few gigs I realized what the problem was. It was adding all new data, so anything that I deleted from the past was still on the external. I want to know how I can just remove everything from the external and start fresh, with what is just currently on my machine. I am afraid if I just start deleting everythign off the external I will delete some critical files or drivers and formatting that would be out of my knowledge to repair. I am using baounceback express as my backup software, the one that came wih the harddrive.