You may want to consider performing a factory reset. While the DNS-321 is powered-up, press and hold the reset button on the rear of the unit for > 8 seconds. This will restore the DNS-321 to the factory default settings, but should not impact your data or HDD formatting. You will, however, lose your user settings (e.g. shares, passwords, etc.).
Upgrading the firmware should not impact your existing data or HDD formatting either.