If you would like to set up your device such that it overwrites existing recordings when it reaches full capacity, do the following:
Navigate to System > General
On this page there should be a feature named "HDD Full"
Click the drop-down and select "OverWrite"
The HDD will now proceed to overwrite the oldest recordings on the HDD. You should notice that your HDD will eventually reach 0 GB of free space, and will remain at this level indefinitely due to this feature being enabled.