External HD for Pro Tools

Hey fellas and ladies,

Well i was wondering what external hard drive gb amount would be enough to help me out backing up some pro tools sessions. I would only be doing let's say one band at a time and then once im done with them erase it all off once we have copies of the songs on a good cd therefore, would 250gb be enough to help back up? my internal has 250 but i wanted a just in case help as well.
Thanks any help would be appreciated! reason why is because i saw very good deals on them at the store, so i wanted to make sure before i dished out the cash<3