There are lots of bus powered USB drives that draw more than the 500 ma at 5V that USB is rated to provide, and many of them come with cables to connect to two USB ports do double the power available (from 2.5 watts to 5 watts). If those drives are connected to two powered ports, they work fine. Part of the USB spec calls for current limiting so that an overload just shuts down the power without harming anything. If overloading a USB port causes damage to the port hardware, the hardware doesn't meet spec and is at fault.
For the curious, the Firewire bus power spec is seven watts, but the voltage can vary over a very wide range and some ports will supply much more.
Last edited: Jan 23, 2007