Thanks to jssilva and K-Berg on this thread! https://developer.apple.com/forums/thread/649956 Trying to help signal boost, as this fix should be easier to find! My update from Catalina was stuck, on iMac (Retina 5K, 27-inch, 2017) for many many hours. I am not a developer, but had this problem, while installing the BIG SUR public release. It got stuck for hours and hours at about 90-95 percent. I even let my iMac sit overnight, no movement from the black screen with apple logo. But then I did as they suggested below, power off, then back on while holding buttons to reset NVRAM.and now everything is working great! Thanks again.