Upgraded to Catalina; Time Machine backup failed
I am no longer able to back up my files via Time Machine onto my external storage. This happened right when I upgraded to Catalina, and (if I recall correctly) it happened the last time I upgraded my OS as well. I am using an iMac Retina 21.5", fairly new.
Usually, Time Machine automatically deletes old files to make room for new ones, but now I simply get a "Backup Failed; There isn't enough space on "(external HD)".
I see lots of advice on how to delete just one or two files to make room, but will that work? Do I have to delete everything now? Is it a problem with the new Catalina upgrade?
iMac 21.5" 4K, macOS 10.15