The power to the External maybe limited by the internal USB hubs. Try plugging the two USB plugs into seperate USB "blocks".
Other than that, it could be your PSU or your motherboard limiting power out of the USB devices.
A powered USB hub would probably fix your external. As for the internal, it's probably just a DOA drive. Internals use 12V rather than 5V (I think) so it's kinda unlikely that they are related.