I have been researching an answer for a few days. The definitive answer seems a little allusive. I am A+ cert. and also have graduated Google training. I don't have any issues performing the upgrade. My question is "Will a 1 T SSD installed in place of the original 1 T HDD that is part of the original Fusion drive work? " As we know, Apple is a little funny about what plays nice with their hardware/software. The logic board has a 32 G Samsung SSProton blade 12x16 pin PCIe 3.0 x 4 interface. Pretty sure this is Apple proprietary. This is the main reason for this question/comment - Will it work if I replace the Seagate 1T HDD (model ST 1000DM003) with a Samsung 1T 860 Pro SSD? I have the sata/thermal cable adapter and frame for 3.5 to 2.5 drive as well as new adhesive strips. Any knowledge I will appreciate. Thanks Dan for the response. I was afraid that would be the solution. Only problem is the expense of the 3.0 x 4 SSDs. 256 is $400. Gotta love Apple.