Hi Zoe,
The backup 'zip' files don't get deleted by any process in Adagio. They can be replaced if a backup will create a new zip file where that file already exists. To remove old zip files, you have to do it manually from Windows Explorer or however you do file maintenance. Perhaps in that one instance, someone removed the old zip files manually.
Adagio DataCare has an option for the number of retained backups. If you use Adagio DataCare for doing backups after integrity checks, then this process will automatically remove older files and only keep the most recent number specified. None of the other Adagio programs do this.
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Regards,
Softrak Tech Support