The answer is yes and no. Let me explain.
WM 6.5 should be capable of acting as a host. I remember seeing a video of an old WinMo phone (was it 5.0 series?) that detected a mouse and KB. I doubt they removed the functionality. Heck! I was able to use my RedFly Celio with my Treo Pro and access everything with it. Anyhow, it boils down to support for a USB standard called On The Go and WM supports this.
You do need to provide a special cable or dongle to enable host functionality. One end is a mini USB connector with the ID pin shorted to GND (look for the pinout online), this is what tells the phone to act as a host and thus expect to manage USB peripherals such as kb, mice and flash thumbdrives; the other end of the dongle should be a female USB type A connector.
Now, the problem with an HDD is that it will require a fair amount of power through the USB port. Usually portable host devices (like the WM phone here) are not expected to drive big powered devices, so you may need to steal power from a external USB charger and connect it to your dongle. A messy affair but doable.