I really want to clean my iMac, as the 1tb drive is 3/4 full. All my music (all 23,000 songs) are in the cloud, as are all my photos. I want to delete the files off my HD, but I am not sure how to do that without losing anything. I would also like to make safety copies onto externals. Is that as simple as locating the libraries and copying over? Not new to Mac, but I am not at all (obviously) comfortable with iCloud. THANKS!