Hard disks were invented in the 1950s. They were originally as large as 20 inches in diameter and had a capacity of only a few megabytes. Hard disks use the same magnetic recording techniques. Hard disks gain the benefits of magnetic storage - magnetic medium can be easily written, erased, and overwritten, and it will "remember" the magnetic flux patterns stored onto the medium for years.
A hard disk consists of numerous parts. Hard disks have an aluminum casing to protect all of the working parts and contain everything. Hard disks contain an electronics board to control the motor to spin the disks or platters and move the read/write head over the surface of the platters. Platters in today's hard disks typically spin at 7,200 RPMs, however, speeds of up to 15,000 RPMs have already been achieved. The read/write head can move from the outer edge of the platters to the center at an amazing rate of 50 times per second. Drives today often have more than one platter.
To determine the speed and efficiency of a hard drive, there are 2 speeds. These are the data rate and seek time. The date rate is the number of bytes per second that the drive can deliver to the CPU. The seek time is the amount of time between the CPU requesting information and the drive delivering the first byte of information to the CPU.
Relative to the head, the platter spins at about 170 MPH. If the read/write head crashes into the platter unexpectedly, due to this speed, permanent damage will usually occur in what is appropriately named a "head crash." If such a crash occurs in the File Allocation Table sector, or FAT sector, all of the data on the drive may become inaccessable. This is because the FAT sector contains the information for the operating system to determine how to read different types of files.