Hard disk drives have become the premier secondary storage device for modern computer systems. Secondary storage is storage that is retained when power is removed. Primary storage on the other hand loses its information when turned off. Computer memory is the most common type of primary storage.
Hard disks use the same technology for storing data that tape recorders use for recording sound. A thin metal disc is coated with iron oxide. When it is subjected to a magnetic field, the particles in the oxide coating align themselves with the field. Computer data is in binary format which means a series of zeroes and ones. If we define alignment in a certain direction as zero and in the opposite direction as a one, then we store data by varying the magnetic field according to the data values.
The read/write mechanism consists of a small electromagnetic head mounted on an arm that is suspended over the metal disc. Electrical impulses sent to this head create the magnetic field that aligns the oxide particles as the disk spins.
The first commercial hard drives were in cabinets that were 1 Â½ to 2 feet across. They were extremely expensive and so only available for large mainframe computers. As technology advanced, disk drives increased in storage capacity and decreased in price and size until today when one can easily hold in one hand a drive larger in capacity than many of the early disk drive cabinets.
But regardless of size or capacity all disk drives operate on the same principle. The oxide-coated disk is called a platter. A drive may have a single platter or it may be a multi-platter unit. The platter or platters is connected to a motor that spins the disc at 20,000 to 30,000 revolutions per minute.
Each platter has a read/write head attached to an arm that can move the head to cover the entire surface of the platter as it spins. The actual head is smaller than a grain of rice. Because the platter is spinning so fast, the head actually floats (in the industry they call this a flying head design) a few thousandths of an inch above the surface.
When the platter stops spinning, the head is designed to move to the side away from the disk surface since were it to contact the oxide coating, it might damage it. The distance from the surface of the disk to the head is many times smaller than a particle from cigarette smoke. If such particles were to get into the drive, the disk surface would be irreparably damaged. For this reason, the platter and head assembly is sealed during manufacture while it is in a specially designed cleanroom.
Today it is common to have a 2 gigabyte drive that has a platter less than 3 inches in diameter. The bits written to the drive are packed incredibly close together. The mechanism that positions the head must be able to move in these extremely small steps. The device that does this is called a voice coil actuator and under command from the electronic control of the disk drive, it can reliably read and write this data.
The disk drive is attached by a cable to a controller board that accepts data from the central processing unit (CPU) and sends the data and control signals to the disk drive. The components of the disk drive must be extremely accurate and robust.
The motor must spin at the exact required speed so that the bits come under the head at the correct rate for reliable reading and writing. It is expected to spin at this speed for years without failure. The head moves at an incredible rate and must arrive at the correct position without over or undershooting and must function as reliably as the motor.
In addition, all of this delicate mechanism must be immune to the shock of moving, bumping, and turning that goes with carrying a laptop computer. The modern hard disk is an amazing piece of machinery especially considering its price.