Backup is failing on a disk that is near its limit with the error message "not enough space". Weird as Time Machine is supposed to delete old backups as needed to store the new ones.
So, I tried to just reformat using Disk Utility and start fresh. That failed with the error message "disk could not be unmounted".
Next, I tried to make space myself by going into the backup folder and deleted a few of the older dated folders. It took forever to delete a folder via Cmd+Backspace, literally over 30min. :-( I tried instant delete (Cmd+Opt+Backspace) but it wasn't faster and failed on a folder with the the message "items had to be skipped" and the tip to unlock. The folder in question is not listed as locked though. On to the Terminal with "rm -rf folder" which resulted in "permission denied". Sudo of the same command just changed the error message to "operation not permitted".
It's also weird because the deleted folders are still shown in Finder - some processes still running. Only about 30GB on a 500GB disk have been freed so far (20 left initially to 50 now).
The especially troublesome folder I tried the "rm" on stands out as it has different permissions. All folders show "drwxr-xr-x@ 3 root wheel" while this one reads "drwxr-xrwx@ 3 root wheel". Some of the deletion processes are still shown in Finder as ongoing.
How can I just reformat that disk and be done with the nitty gritty? Second best, how can I weed out by hand and delete some of the older backups, getting past the above snags?
System: Catalina 10.15.6