I'll be buying a new iMac after Christmas, and want to add an external drive. Reason is I think I can save money instead of paying Apple for a internal 1TB Fusion or SSD. Unsure if it will be too difficult for me to move OS to external SSD and use internal 7200, or just add Thunderbolt SATA3/enclosure SATA3 7200 hdd for storage. Lots of photos and music. I'm a desktop guy. I can follow directions easily, but if the directions are missing a single step, I'm lost. Its been a long time since I have used a Mac, since way back in the 80's. Thanks for any help in advising me.