Export Backup Incremental

Hi,

I have written a script to export a full backup and copy it to an external location regularly.
Can I leave the previously exported data inside the temporary export directory? What is the official policy? Can this create problems? Afaik the wiki does not say anything about that.
When I try that from the admin interface, it gives an error saying the export directory must be empy. The CLI does not.
Maybe that would even be more efficient, because only the changes to the last time have to be exported.

Thanks for your answer.