Chosen Solution
My 1TB seagate HDD failed and I replaced it with an EVO 860 SSD 1TB, using iFixIt kit and OWC thermal sensor. Upon power up (w Cmd-R) I first used Disk Utility to erase the SSD, then ran time machine. It kicked off a 6.5 hour job restoring and I went to bed. When I came back in the morning, the machine seemed asleep and screen was black. Unresponsive to keyboard or mouse. Hard-rebooted w power button, and upon restart I got the gong and a fully black screen (no images visible with angled flashlight). After about 1 minute fans kicked into to full power. This behavior has continued after all the following experiments: Rebooted Cmd-Opt-P-R. No change following. Rebooted again Cmd-R: no detectable difference (still gong followed by black screen and full fans after about a minute). I let the machine run with full fans for about 10-12 minutes, but no change. As before, forced shut down by holding down power button. I’m thinking of next re-opening to check connections — BUT I had fine video upon the initial power up w the SSD installed, and I lost this apparently fine operation following the Time Machine restore, though I wasn’t there after 6.5 hours to witness how it actually concluded the restore. So… a connections issue after operating fine seems surprising to me. To others? I was also thinking upon reopening, with everything still connected, to boot and try to see the LEDs — not sure if they can be visualized. I don’t have a thunderbolt to VGA adapter — so I can’t see if it’s able to drive an external monitor even while the iMac remains black. Confession: upon reconnecting after the SSD installation, I let the screen drop a little and the LCD power display cable popped out of the connector mounted to the logic board — damaging the plastic walls of the connector. I was able to straighten the pins and get the plug to reinsert fully (as best I could tell). I secured the cable and connector with tape (low tech, but I was thinking it was a very low stress job). So… could heat cause regular cellophane tape to come loose? Does it make sense to anyone that the display power cable could have become loose after it had worked? Any suggestions on trouble shooting this mystery will be very much appreciated!
Try connecting an external display to see if the system is running at all. If you don’t have a second display but have an another Mac see if you can remotely access it over the network or try Target Disk Mode.
