2015 iMac 21.5” Black Screen after installing new hard drives
Using the IFIXIT example “iMac Intel 21.5” Retina 4K Display Blade SSD Replacement, I was able to install a WD Black 1TB blade into a Sinetech adapter, which was intended to be used in Mac laptops and Mac Pro of 2015 (and other years). I filed down slightly the metal area where the screw goes and it made for a perfect fit onto the iMac. On my way out I replaced the original HDD with a 1TB WD Blue SSD.
After finishing the installation (but had attached the screen only with tape across the bottom, I was careless and the screen fell open popping the cables loose. The display data cable would not properly attach following the incident. I ordered a date cable and installed it but still no picture. There are three small green lights that I can see when lifting the screen up. There appears to be a fourth light not on, on the end nearest the top of the screen.
The iMac works with an external monitor attached although the fan is running very fast. It is booting into Catalina from the blade,
Before I proceed further, I want to seek some advice. Thank you in advance for any help/advice you can provide.