I have a Time Capsule that was used for backing up several offline macs, when the local drive sizes were relatively small. (Lets say theres two old backups called "iMac" and "iBook".
Currently it backs up two relatively new devices (lets call them MacBook and Mini).
It has become full in the last year. I know that Time Machine discards older backups to keep the most current, but how does it decide what data to keep between the different systems?
Best Answer
Space will be reclaimed by Time Machine only using the older content from backups of the same computer. Consult this archived page from Pondini:
With a Time Capsule or any network drive used as a disk for Time Machine, every computer backup is stored in its own sparse bundle disk image. This is how multiple computers can be backed up to one location. Within each sparse bundle disk image, linking is used to minimize the storage required for files or folders which have not changed between backups.
This archived page from Pondini explains how to manually delete from backups and also explains the structure.