I thought it could be used for an external hard drive, but I was wrong. I have had that router for a year now. The USB port is just for configuration. It can't be used for anything else. What a let down for such an expensive router.
With the latest firmware (1.21) it can be used for sharing an external USB hard drive on your network.
Make sure you have the latest firmware for the 655 and in the setup go to the USB settings and make sure it's set for Network USB and then download and install SharePort Utility and it should find your USB hard drive just fine.