This article explains that Unitrends Free relies on automated purging to ensure space for new backups.
Unitrends Free always retains the most recent successful backup of each protected virtual machine in your environment so that your data is always available when you need it. However, to ensure space for new backups, the appliance automatically purges the oldest backups as needed when the appliance runs out of backup storage. Note that the most recent successful backup group of each virtual machine will never be purged during this process.
Additionally, you can always expand your backup storage to prevent purging from occurring. For more information, seeÂ StorageÂ in the Unitrends Free Administrator's Guide.