Time Machine backup doesn't overwrite old backups automatically, instead prompts there's no more space left. Can it just overwrite old backups automatically?
By default, the Time Machine function is set to unlimited capacity so it won’t overwrite any old backups you had before. If you would like to change this setting please follow the steps below:
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Open Hybrid Backup Sync
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Select Backup Server from the left sidebar and then click on Time Machine
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Please adjust the “Capacity” setting to your desired value (in GB). Once this allocated space fills up on your NAS, the older backups will automatically be overwritten.

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Hit Apply to save your changes