I have two 1 TB "Buffalo" hard drives from Other World Computing which I use to back up my computer. One I keep unplugged and stored separately. They simply plug into the USB port of my Macbook Pro. I am considering getting a third external hard drive for further redundancy. I see on the OWC site some external drives are BUS powered like my current two drives. However, others are externally powered by plugging into the household current. This seems like more of a hassle than the simpler BUS powered drives. There must be some reason externally powered drives are made?