The vast majority of desktop and laptop computers contain both random access memory (RAM) and magnetic disk drives (hard drives). Each of these hardware components hold data for the user, but they have unique characteristics. For one thing, the electronic RAM circuits of main memory, while more expensive than magnetic disk storage for their storage capacity, are able to manipulate data very quickly. Magnetic drives are physical devices that store and retrieve data somewhat more slowly.
However, magnetic drives have unique advantages: unlike main memory, they can store information even when the computer is turned off. Magnetic disk storage typically has more capacity than main memory, and the data on magnetic hard drives can be moved from computer to computer. Being aware of these differences can help a user understand how computer hardware works "under the hood."
Magnetic disk storage is permanent. This is probably its chief advantage over random access memory, or RAM, used for main memory. RAM, made up of electronic chips that require constant power, is described as volatile: when the user turns off the computer, the information contained within RAM is lost, as though it were erased.
Magnetic disk hard drives save information on physical media--one or more rotating circular platters--where the information remains until the user chooses to revise it or delete it. Even when the computer is turned off, the data remains on the hard drive until the computer is turned back on. The information saved on a hard drive is available for years, provided the device does not fail or become damaged.
Magnetic hard disks have come a long way since the inception of personal computers. Hard drives held just a few hundred megabytes or less in the early years. Now some hard drive storage units hold terabytes of data, a million times more capacity than a megabyte.
Memory installed into computers today can hold up to several gigabytes of information, with each gigabyte holding roughly a thousand times more than a megabyte, but a thousand times less than a terabyte. This is far less data capacity than the magnetic hard disk installed in these same machines.
This extra capacity also allows the magnetic hard disk to be used as supplemental "virtual" memory when main memory fills to capacity. But since magnetic disk storage is not as fast at storing and retrieving information as electronic RAM, the computer tends to slow down if not enough main memory is available for the tasks at hand.
Although internal hard drives require some effort to access and remove, external hard drives offer portability. Since magnetic disk storage retains information even without power, a user with an external hard drive can transport the magnetic disk storage, along with their data, from computer to computer. RAM loses data the instant it's cut off from its power supply, including whenever the computer is turned off or the electronic RAM chips are removed from the computer.
Even internally installed hard drives can be moved from computer to computer on occasion, although this requires extra effort. If a user decides to build a new computer, the hard drive can be moved to the new computer, where the data that remains stored on the hard drive can still be retrieved. These hard drives can also be used as a backup storage device.