I was on a startup drive with a copy of Catalina. I think Apple may have made it difficult to put a beta on a boot drive. I only had luck by getting another fast non-spinning drive. I had no problem installing on a drive I just formatted. Apple really needs to have a simple way of explaining apfs in its various flavors. You then have to decide. One thing I'm dimly understanding about apfs is that it puts your data wherever there is room. Including volumes on other hard drives or flash or ssd. There's no app yet that gives you a useful idea of what drive has what on it. apfs does some great things, but they need to communicate with the average Mac user. I would love to see some documentation, like the privacy paper and so on. I have made mistakes handling it. Suddenly I see only my data volumes. With that, Big Sur is still beta until all the OSes are finished. As apfs seems still to be. Anybody know what I can read from Apple or anyone else?