I have a iMac with a 500G HD that is almost full.. I want to add an external drive and move all my photos to it.. I have over 300G of photos and shoot in RAW.. I import in iPhoto but do editing in Photoshop Elements 12.. I backup using Timemachine and would like to setup the HD so that its works like the original HD.. Any suggestions and directions on how to do this would be appreciated..