I have a late 2013 iMac that I just repaired tonight. The 3gb HDD portion of the fusion drive was dead, as well as the clutch for the hinge (long since dead, was coping using a block to support it). Although it is my understanding that the hinge could have been covered, I figured as long as I was digging in there to replace a dead hard drive, I would just fix the hinge. Had I known how annoying it was to wind the spring while trying to screw in the replacement bar, I may have opted to have it done for me. A good learning experience, but my poor thumb is still sore from that task. Back to the fusion drive. My iMac had gotten so slow as to be completely unusable, and after some attempted fixes and debugging, I ended up splitting the drive, and found the 128GB SSD portion was good, and the HDD portion was bad (The HDD was not making any odd telltale noises at that point, nor showing any SMART errors). After splitting them, and installing Sierra on the SDD, everything was working fine and the HDD portion was obviously the dead culprit. I replaced the dead HDD with a 1tb SDD. Now, my question is, is there any reason I would wish to fuse those back together in to a fusion drive before I go too much further? As of right now, I am thinking leave the 128GB SSD for os/apps and use the 1tb SSD for data.