A traditional hard disk drive (HDD), sometimes referred to by its abbreviated names hard disk or hard drive, is a form of magnetic mass storage used primarily in computers, but also in some portable music players, video cameras, DVD players, and video game consoles etc.

Solid state drive (SSD) technology can also be used as hard drives, where data is stored on an array of chips attached directly to the circuit board. These are also found in music players, but also in mobile phones, tablets, and many modern smart devices.

How data is stored on a hard drive

Components of a hard disk drive

Components of a magnetic hard drive

A traditional magnetic hard drive is composed of two essential components.

A mechanical component

  • The drives are double-sided circular trays, most often made of aluminium, covered by a magnetic layer where data is stocked and organised. These drives are placed around a rotary axis driven by a spindle motor. The speed of the rotation varies according to the brand and model of the hard drive at generally between 5,400 and 15,000 rotations per minute.
  • The read head/write head is another mobile component of the hard drive. They are steered by an actuator that is driven by a second motor. Their pivoting motions allows them to sweep the entire surface of the drives. The role of the arm is to position the heads on a certain path to have access to the information. It is important to note that the read heads are never in direct contact with the magnetic surface of the hard drive. Even light friction or a speck of dust is sufficient to damage a drive.

An electronic component

Though the mechanical components are the most visible on a hard drive, the electronic component is just as important, because it handles the data transmission as well as procession and commands between the motherboard and the hard drive. Each hard drive has a microprocessor and an associative memory held on a printed circuit board (PCB). A signal processor handles the conversion of electric signals to digital signals.

Current standards

  • Technology: The magnetic hard drive represents today's predominant technology. Magnetic tapes and optical storage (CD/DVD) are most often used for data storage and backup in a professional environment.
  • Size: As well as the storage capacity of hard drives, the physical size of the device has also undergone significant evolutions during the last 50 years. The RAMAC was the size of two refrigerators, while the majority of today's hard drives measure 3"1/2 (3.5 inches, or 9 cm). The standard size for laptop drives is 2.5 inches. The boom in MP3 players and other portable devices have also contributed to the development of microdrives that measure only 1 inch!
  • Interface: The interface is an essential component of all hard drives because it links the motherboard to the hard drive. The type of interface, among other factors, determines the speed of information transfer. Just like the size and capacity of hard drives, interfaces are in constant evolution. For the sake of simplicity, we will only discuss the most common ones:
    • SATA (serial ATA): the most common interface since 2005, it allows for faster transfer rates than the ATA,
    • SCSI (Small Computer System Interface): this interface allows the connection of various peripherals via an adaptor or SCSI controller.

How is data stored on a hard disk?

Data is stored on a hard drive in binary code, using 1s and 0s. The information is spread out on the magnetic layer of the disk(s) and are read or written by the read heads that 'float' above the surface thanks to the layer of air produced by the ultra fast rotation of the disk.

In writing mode, an electrical current travels via the heads and modifies the surface of the electric field by inscribing a 0 or a 1. In read mode, the process is reversed: the magnetic field transmits an electrical current to the read head, and this signal is then translated into a digital signal readable by the computer.

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