There are many people who struggle to run their external HDD in their OQOs. Why? Because the OQO has only 1 USB port and some of the external HDDs require two USB ports to power themselves.
Some people think that the OQO does not provide enough power via its USB port. No, that is not the case. The OQO models are fully USB 2.0 compliant and provide upto 500mA of power to the USB device for them to work. If your device requires more than that or if it requires two USB ports, then you are out of luck.
Make sure that the peripheral that you are buying works with the power provided by the OQO and uses only one USB port.
[related OQO forum post]
February 13, 2009