External drives usually require a power source independent of a computer.
The portable external drives take power from the bus they are on (USB or IEEE 1394,) so no power brick.
Thing is, if it is a portable drive to a portable computer on battery, it will shorten the battery time.
Remember some motherboards are set up to spread the power over several ports on the bus, so a heavy draw from a portable drive will pull signal and power away from other devices.