I have successfully replaced the internal 3TB Seagate HDD with a 4TB Samsung 860 Pro using a 3D printed 2,5” to 3,5” adapter. No additional thermal sensor is required, the fans are working as they should do. Quiet in idle and noisy at load. Tested with two iMac 27” late 2015 version 17,1. Also the Fusion Drive PCIe SSD is now available as a separate and fully functional additional drive with 669 mb/s write / 1710 mb/s read.