What data are you looking to backup? The statistics collected by Harvest are not stored in any Harvest directory. They are saved in the Graphite database which is located under /var/lib/carbon/whisper by default. In my environment, I created an iSCSI LUN from an ONTAP cluster and mounted it under /var/lib/carbon/whisper. That way my Graphite data is protected.
Ah, I see why you are asking now. Are you running the NAbox OVA? I am not. Nevertheless there should be local directory that at the top level it has subdirs "carbon" and "netapp" inside that directory structure are all the Graphite database files (its a pretty deep directory structure). Those are the files that actually contain all the performance information collected by the Harvest pollers. If you wanted to back up Graphite data, then that would be the data to back up.