For context, I have a 1TB internal SSD with 350GB used, and a 4TB external HDD with just over 3TB used. I've been backing up to an 8TB external HDD for about 3 years with no problems until now.
My understanding of Time Machine backups is that they are incremental and what gets backed up are the changes. I also thought that it would delete old backups once the disk filled up. But, lo and behold, I ran into an issue where it "doesn't have enough space to back up."
So, 2 questions. Shouldn't Time Machine delete old backups to make room for this new one? And why is it suddenly requiring 3TB, if it's just storing the changes? The only thing I can think of would be if I moved a bunch of files between drives, but I didn't.