Overnight my macOS Catalina server filled up its disk thanks to #CarbonCopyCloner creating a copy of the external clone drive's Data partition to the internal drive's Data partition. It's never done that before. I don't know whether some mount point got messed up and confused Catalina or what. Anyway, thanks to ChatGPT's help I located and fixed the problem. It's even making suggestions to configure CCC so it doesn't happen again. So weird, but then, weird problems find me.