Chosen Solution
I have a late 2015 iMac 27" 1TB Fusion Drive. I switched out the Seagate 1TB HD with a Samsung 1TB SSD and also switched the apple 32GB PCIe blade SSD with a Samsung 128GB SSD. Before replacing the LCD screen I turned the computer on at the mains (started up without pressing the power button). I proceeded to reinstall OSX Sierra from a USB Flash Drive, while this was going on I reattached the LCD screen. Once OS-X was reinstalled I then migrated my info from my backup disk. All was working fine. I powered down my iMac but now it won’t power back up. I’ve checked the power wire from the power button which is fine and connected correctly, there is 1 LED lit on the logic board when plugged into the mains and all connections appear intact and fitted correctly. Any suggestions?
To start with you’ll need to pull the M.2 drive as Apple’s driver won’t support it. You’ll need to install the custom Apple PCIe NVMe SSD drive. Did you first break the Fusion Drive set? As you can’t reuse the drives until you do. This also can effect things as the settings are also saved in NVRAM. You can’t create a Fusion Drive with with two SSD’s as the OS is unable to deal with this config. You’ll need to leave the system setup with two discreet drives. You also need to install the in-line thermal sensor to replace the one that was in the HD you pulled. This is what you’ll need: OWC - In-line Digital Thermal Sensor for Hard Drive Upgrade for 27" iMacs 2012 & Later
