Backing up - help!

I just got a Macbook Pro.
I'm trying to move Mobile Me to iCloud
It said to back everything up so I went into Time Machine to do that. It won't let me, saying I don't have enough room on my external hard drive.
I don't know anything about this so don't know what I'm looking at but I have never come close to filling my external hard drive before. It looks like I have my old Macbook backed up, my Mac book Pro wanting to back up (in process) but maybe I have backed up a copy of my external hard drive as well?

Is that possible? WHat should I be seeing when I open my EHD and what should I not be seeing?
Help!