This will depend on a few factors i.e. that is whether or not your accessing the attached HDD through the WAN interface, LAN and / or WLAN. The speed / capability of each interface / adapters etc. will play a role in the throughput achieved. Providing that the HDD is USB 2.0 compliant you should be able to achieve a maximum throughput of approximately 480Mbps between the HDD & NB620Ws USB 2.0 connection.