We have a Late 2012 iMac in the shop that needs the 3TB hard drive replaced. Turns out this model has a Fusion drive with an SSD that is on the backside of the logic board.
The drive has been swapped (new WD Blue 3TB), but we are having issues getting the Fusion setup, and getting OS Sierra loaded. I have tried researching the Fusion drive and what needs to be done, but all I can find is people trying to replace the HDD with an SSD, and other "projects".
When we go to Disk Utility from a Sierra flash drive, it does not give me any options to rebuild the Fusion drive or anything like that. If I plug up the original HDD to another Mac, it does tell me it needs to rebuild it, so that confirms it is a Fusion drive.
How do we properly replace the HDD, and either get the system booting as-is (is the OS on the SSD?), or reload OS Sierra on it? We are currently working on doing a TimeMachine backup on the system, but it is about 2.2TB of data...