I recently purchased a 1.5 TB Seagate external hard drive for use with my Macbook. My Macbook's HD is full, and I would like to move some files (iTunes, pictures, videos...) onto my hard drive. How would I do this? Also, say I move Pictures onto the external HD. Next time I download or import pictures, how do I know they will go to the external HD rather than to the Macbook's drive?