So I have an external hard drive that I use as a time machine backup for my iMac. However, I recently got the 256 gb (or how ever many) 15" Macbook Pro with retina and I will need an external hard drive for that. Not only for Time machine, but for story movies and stuff on it. Is it possible to use the same external hard drive from my iMac and not use Time Machine? What is the best thing for me to do?