I assume you are talking about a new 4th Generation Nano iPod? If so, this is a commonly discovered change in the design of the Nano. Older iPods used to charge off either USB or Firewire connections (they are different pins on the dock connector). Many speaker systems, car stereo iPod interfaces, etc. that can charge an iPod do so through the Firewire power pins of the iPod.
New iPods, including the newest 4th Generation Nanos, no longer are able to charge off the Firewire power pins of the dock connector and can only charge off USB power to the USB power pins.
So your iGroove likely charges through the Firewire pins, not the USB pins.
Patrick
