If you check his (@OKonnel) signature you can see he owns an iMac14,2 with an 780M. I knew it was a typo from the beginning and I was not starting a tour de horizon about ancient NVIDIA graphics cards.Support for Nvidia maxwell cards (such as the gtx 960 & 980 models) stopped with High Sierra, when NVIDIA stopped producing/releasing its web drivers. Whoever wanted to upgrade to mojave and higher, in mapros that is, had to buy AMDs Polaris and Vega cards. Did you actually mean a gtx 780? To echo Ausdauersportler there hasn't been a gtx980m for apple's laptops or iMacs.
I cannot answer with regard to your specific startup problem. I use Catalina and Big Sur in a 5,1 Mac Pro via open core.
There exists in fact a GTX 980M (MXM) card which runs fine in these iMacs using an MXM slot, unfortunately - as we both know - only with non macOS system offering driver support. Apple laptops had ?never? used MXM cards?
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