Ah, the wheel. Without it we wouldn't have cars--or hard drives. And the truth is, storage engineers love things that spin. Before the hard drive, magnetic tape on reels spun frantically on mainframe computers. Problem was, if the piece of data you wanted was at the end of the tape and you were at the beginning, you had to endure a seemingly interminable wait for an entire spool of tape to spin onto the take-up reel before you could get to the part you wanted.
By comparison, magnetic disk recording must have offered quite the epiphany. With magnetic disk recording, you can move the read/write head more-or-less directly to where the data is--allowing you random access and a much quicker process than waiting for a thousand feet of tape to spin under the read/write head.
Hard Drive Defined
A hard drive is a storage device that rapidly records and reads data represented by a collection of magnetized particles on spinning platters.
If a computer's CPU is the brain of the PC, the hard drive is its long-term memory--preserving data programs and your operating system even while the machine is asleep or off. Most people will never see the inside of a hard drive, hermetically shrouded as it is in its aluminum housing; but you may have noticed an exposed PC (printed circuit) board on the bottom.
This PC board is where the brains of a drive are found, including the I/O controller and firmware, embedded software that tells the hardware what to do and communicates with your PC. You'll also find the drive's buffer here. The buffer is a holding tank of memory for data that's waiting to be written or sent to your PC. As fast as a modern hard drive is, it's slow compared to the data flow its interface is capable of handling.
If you took apart a desktop hard drive, you'd typically see from one to four platters, each of which would be 3.5 inches in diameter. The diameter of the platters used in hard drives for mobile products vary from as little as 1 inch for drives that are used in music players and pocket hard drives to the 1.8-inch and 2.5-inch platters typically used in notebook hard drives. These platters, also known as disks, are coated on both sides with magnetically sensitive material, and stacked millimeters apart on a spindle. Also inside the drive is a motor that rotates the spindle and platters. The disks in hard drives used in notebooks spin at 4200, 5400, or 7200 revolutions per minute; desktop drives being manufactured these days spin their disks at 7200 or 10,000 rpm. Generally speaking, the faster the spin rate, the faster data can be read.