Black screen while booting after replacing fusion Hd with ssd
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!