External Hard Drives. An external hard drive is usually used for two main purposes: to act as a storage back-up system and/or to provide additional storage. External hard drives can store huge amounts of data and files, which saves your computer’s internal hard drive from having to store this data and possibly running slower. External hard drives are not that expensive for what you get. For about $100, you can get a terabyte of extra space – that’s enough for about 230 DVD movies or 750,000 photos or MP3s.