The hard-disk drive is so common that most computer users take it for granted as a natural part of a personal computer. But now, the hard drive has a challenger for its longtime role as the principal storage device in computers. It's called the solid-state drive, or SSD, and it has begun to show up in some big-name notebook computers. Hard-disk drives, or HDDs, are mechanical devices. They work by recording data on a spinning magnetic platter or platters. By contrast, solid-state drives are made of chips and have no moving parts. They are close cousins to the so-called flash memory used in digital cameras, cell phones and smaller-capacity music players. They record data to special memory chips that retain their contents even when the device is turned off.