Hard drives, the rigid platters holding vast amounts of information. The platters spin at
speeds of 5,400 to 15,000 revolutions per minute, and “read/write" heads scurry across
the platters to move information into RAM or to put new data on the hard drive. The
drives, which can move dozens of megabytes of information each second, are permanently
sealed in metal cases to protect the sensitive platters. A drive’s capacity is measured in
gigabytes, and only the most basic desktop computers today ship with less than 250 gigabytes of hard-drive capacity. The drives almost always hold the operating system for a computer, as well as key software programs (PC World, 2007). Although nearly every computer has a built-in hard drive, external drives that plug into computers using universal serial bus (USB) or other ports are
increasingly common. Many external drives are powered through the USB port, meaning
the drive need not be plugged into a traditional electrical socket.