I have just upgrades from OS X Yosemite 10.10.5 on an old 2008 iMac to Big Sur 11.01 on a 2017 iMac. The problem I am having is installing fonts from any old Mac. When installing into Font Book on Big Sur, I get no dialogue box to confirm installation. The fonts simply don't appear anyway. Also when I select the fonts I am about to install (from a memory stick) and select validate files, they all flag up as serious errors. Finally, on Font Book, top left originally had 'All Fonts, Computer and User' now only All Fonts appear. Any help is greatly appreciated.