The following holds true for the incremental backup functionality on the DNS-343 and I believe this also applies to the DNS-323 (someone please verify):
If you plan on filling a significant portion of the source HDD with data, keep in mind that the on-board incremental backup functionality does not have the capacity to remove files in the backup HDD that are deleted from the source HDD. That said, the backup data repository will get progressively larger over time, far exceeding the size of the source data and can possibly fill your backup HDD if you're storing a lot of data. As the backup HDD fills up, you will either have to manually delete unwanted files or periodically sync the source/backup HDDs using a third-party backup app.