回答“iMac won't boot after RAM upgrade”Today is December 30, 2020. My iMac 27” is 2019 Model with Inter i9 processor and 8 GBx2 RAM. I purchased 16GBx2 2666 MHz OWC RAM and installed following instruction video in Youtube. I couldn’t restart my iMac for many times. Later on, I pulled out the original two 8GB RAM and tried, time iMac started normally, with 32 GB new RAM. Then, I shut off iMac, re-installed the two 8GB RAM back, this time iMac started normally. Checked “About this Mac”, it showed “Memory 48 GB 2667 MHz DDR4”. I don’t understand why it is like this. But I am happy it works as expected. BTW, I purchased OWC RAM because some websites says Apple use OWC RAMS and I didn’t open Mac to check it before I purchase it from Amazon. It turned out original RAM is SK Hynix.