I have a shared external disk which rapidly filled with time machine backup preventing further backups.
Why is this happening and how can I prevent this?
There are 3 macs backing up to the shared disk, and one, my partner's MacBook Air running High Sierra with 120 GB of disk, consumed most (600GB) of the 1 TB external disk. The other 2 are a Mac Mini Server running Catalina with the external disk attached which consumed 60GB, and a development MacBook Pro running Catalina which used 337 GB.
When I looked at the MacBook Air time machine, it asked if we wanted to start a new time machine backup! It said timemachine had completed a verification and needed to start anew. I think this has happened many times.
On the MacBook Air, I turned time machine off then opened the external disk, selected the SparseBundle of the backup and deleted it. After about 15 minutes of saying deleting 0 items, it stopped and said the delete couldn't be completed due to permissions. Thankfully there is now 603 GB of free space on the external backup disk.
So I have the space back, but I am concerned that re-starting the time machine backups on the MacBook Air will repeat the rapid fill. It would be good to start with a fresh Sparsebundle for the Air on the external disk in case the current one is corrupt. I don't want to re-format the disk and lose the other backups. The Air is only used for browsing, photos, and email so no large scale data creation and churn (the photos don't get edited or processed).
The external disk is formatted as Mac OS Extended (Journaled). The server to which it is attached can't delete the Air's sparsebundle due to access permissions, although having re-started the Air's time machine backups the old sparsebundle is now shown as purgeable.
The Western Digital external disk is connected via Firewire800.