The factory installed DNS-320 and DNS-325 firmware do not support 3TB HDDs. If a 3TB HDD is formatted with a DNS-320 running firmware version 2.00 or earlier or DNS-325 running firmware version 1.01 or earlier, the HDDs will format to 1.96 TB of usable storage space.
To format 3TB HDDs to their fullest capacity, upgrade the latest firmware on the DNS-320 (v2.01 or greater) and DNS-325 (v1.02b03 or greater).
The firmware must be upgraded before formatting the HDDs.