I have a iMac 21.5 late 2012 Yosemite 10.10.3 2.7 GHz Intel Core i5 8 GB 1600 MHz DDR3 Recently the HD packed it in. I have an authorized, reputable service tech replacing it with a 3 party manufacturer replacement (yes we know about the sensor concern and have addressed that). I have all my data backed up on an external HD using Time Machine. When I get the computer back with the new HD, what do I do? I mean I don't know how the new HD is formatted or how I will instruct it to copy the data to the new drive if there is no OS on it. This may sound dumb because there is a lot of dumb taking up space in my head. So if you could please explain in simple language, I would be very appreciative.