A hard drive contains a spinning metal disk. The surfaces of the disk have a magnetizable coating. An actuator arm with a read/write head on its end is passed over the surface of the spinning disk.
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Anatomy of a Hard Drive

The operation of a computer requires some means to store programs and data. The programs and data could be stored in some type of semiconductor memory. RAM (Random Access Memory) is a type of semiconductor storage that loses its data when the power is removed. ROM (Read Only Memory) doesn't lose its data when power is removed, but data can be written to a ROM only once.

Flash memory is a special kind of semiconductor that doesn't lose its data when power is removed, and it can be read and written to many times. but flash memory is expensive. A hard drive is basically a low-cost mechanical means of storing a large amount of data that isn't lost when the power is removed, and can be read and written many times.

A hard drive contains a spinning metal disk. The surfaces of the disk have a magnetizable coating. An actuator arm with a read/write head on its end is passed over the surface of the spinning disk. The hard disk drive contains circuitry to control the position of the actuator arm and provide a signal to the read/write head.

The disk spins at 7200 rpm. When the disk is spinning, the read/write head floats on a thin layer of air slightly above the surface of the disk. The space between a read/write head and the surface of the disk is around 50 nanometers, about 1/2000 the diameter of a human hair.

- You should never open a hard drive's sealed case because a speck of dust is bigger than the gap between the disk and the read/write head. If dust enters the drive, it can have catastrophic results.

To store a binary 1, the read/write head magnetizes a tiny area of the disk's surface. To store a binary 0, the read/write head magnetizes an area with the opposite polarity. To read data, the read/write head detects the polarity of the magnetized areas.

In the early days of computers, the electronic control circuitry was on a separate circuit board that was plugged into an expansion slot on the computer's motherboard. Today, hard drives have Integrated Drive Electronics (IDE). The control circuitry is mounted on the hard drive.

The magnetized areas containing the data follow concentric circles around the disk called "tracks". The tracks are divided into pie shaped "sectors". Tracks closer to the center of the disk are smaller than tracks closer to the outside edge. However, every sector still contains 512 bytes of data. This is accomplished using a method called "precompensation".

A hard drive uses both sides of the disk, with an actuator arm and read/write head for each side. Large capacity hard drives may contain as many as eight disks, referred to as "platters", with a total of 16 read/write heads.

Hard drive with stepping motor

Early hard drives used a stepping motor to position the actuator arm so that the read/write head was over the desired track. When the hard drive was turned off, the read/write heads moved to a special location on the disk called the "parking" area. In a sudden power failure, the stepping motor couldn't park the heads. The heads would "crash" into the disk, resulting in a catastrophic failure.

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