How does Microsoft's UVCView utility see the external drive? http://www.users.on.net/
Does the drive spin up? USB 2.0 ports are current limited to 500mA, wheres a typical 2.5" drive requires about 850mA during spin-up. Some ports can handle the extra current demand, others can't. In the latter case you can use a USB Y-cable to pick up power from two ports.