Chosen Solution
After carefully removing screen and replacing the fusion with a mercury ssd, the iMac boots with a black screen and takes five minutes to get to sign in screen. Thereafter it works perfectly. I tried booting from and external ssd drive with clean Mojave os, with same result. After the iMac runs for an hour or so and is shut down, I can reboot with same black screen but only takes a minute to reach sign in. Any suggestions!
I promise you the 2012 systems (and all others) with a Fusion Drive are in fact dual drived! It has a SATA HDD and a PCIe Blade SSD. Each uses its own connection to the logic board: iMac Intel 21.5" EMC 2544 Hard Drive ReplacementiMac Intel 21.5" EMC 2544 Blade SSD Replacement Here’s how one can create a Fusion Drive in an older Mac system https://www.macworld.com/article/2014011… Here we’re using two SATA based drives (HDD & SSD). So it sounds like the flash drive is still present. Update (03/31/2019) The problem you are facing right now is the blade SSD is the still active boot drive looking for the HDD mate of the Fusion Drive set. Basically, the Fusion Drive was not properly broken up before swapping out the SATA HDD for the SATA SSD. So what to do?? Putting back the HDD and then breaking the Fusion Drive following this: How to split up a Fusion Drive. Then the blade SSD can be disabled by reformatting it and leaving it as is. The alternate way is to remove the blade SSD. Often I find people are willing to go with a better blade SSD in place of the small SSD Apple installs with it’s Fusion Drive setup. A PCIe/AHCI SSD drive in this series.
