The status LED on an AirPort Extreme may be lit if certain self-powered USB devices (for example, printers, hubs, or a storage device) are connected and powered but the external power supply of the AirPort Extreme is off (unplugged from the AirPort or the wall outlet). The AirPort Extreme is not operating correctly in this state.
Note: You should also avoid using any USB storage device that draws power from multiple USB ports (with a Y-adapter, for example). Use an external power source for the device and a regular (not Y-cable) to connect to the AirPort.