Hard disk

                              Hard drive

The hard drive is the component which is used to permanently store data, as opposed to RAM, which is erased whenever the computer is restarted, which is why the term mass storage device is sometimes used to refer to hard drives.

The hard drive is connected to the motherboard using a hard drive controller which acts as an interface between the processor and the hard drive.

Structure

 A hard drive is made up of not just one, but several rigid metal, glass, or ceramic disks, stacked very close to one another and called platters. 

The disks turn very quickly around an axle (currently several thousand revolutions per minute) in a counter-clockwise direction .

The data is stored in the form of 0s and 1s (called bits).

Hard drives hold millions of these bits, stored very close to one another on a fine magntic layer a few microns thick, which is covered by a protective film. 

They are read and written using read heads located on both sides of the platters. These heads are electromagnets which raise and lower themselves in order to read or write data. The read heads are only a few microns from the surface, separated by a layer of air created by the rotation of the disks, which generates a wind of about 250km/h (150 mph)! What's more, these disks are laterally mobile, so that the heads can sweep across their entire surface. 

 

However, the heads are linked to one another and only one of them can read or write at a given moment. The term cylinder is used to refer to all the data stored vertically on each of the disks.

This entire precision mechanism is contained within a fully airtight case, as the smallest particle can degrade the disk's surface. This is why hard drives are closed shut with seals, and the warning "Warranty void if removed", as only hard drive manufacturers can open them (in particle-free "cleanrooms").

How it works

 

The read/write heads are said to be "inductive", meaning that they can generate a magnetic field. This is especially important in writing: The heads, by creating positive or negative fields, polarise the disk surface in a very tiny area, so that when they are read afterwards, the polarity reversal completes a circuit with the read head, which is then transformed by an analog-digital converter (ADC) into a 0 or 1 which can be understood by the computer.

 

The heads start writing data from the edge of the disk (track 0), then move onward towards the centre. The data is organised in concentric circles called "tracks", which are created by  low level formatting.

The tracks are separated into areas (between two radii) called sectors, containing data (generally at least 512 octets per sector).

  

The term cylinder refers to all data found on the same track of different platters (i.e. above and below one another), as this forms a "cylinder" of data.

Finally, the term clusters (also called allocation units) refers to minimum area that a file can take up on the hard drive. An operating system uses blocks, which are in fact groups of sectors (between 1 and 16 sectors). A small file may occupy multiple sectors (a cluster).

On old hard drives, addressing was done physically, by defining the position of the date from the coordinates Cylinder/Head/Sector (CHS). 

Technical specifications

• Capacity: Amount of data which can be stored on a hard drive.
• Transfer rate: Quantity of data which can be read or written from the disk per unit of time. It is expressed in bits per second.
• Rotational speed: The speed at which the platters turn, expressed in rotations per minute (rpm for short). Hard drive speeds are on the order of 7200 to 15000 rpm. The faster a drive rotates, the higher its transfer rate. On the other hand, a hard drive which rotates quickly tends to be louder and heats up more easily.
• Latency (also called rotational delay): The length of time that passes between the moment when the disk finds the track and the moment it finds the data.
• Average access time: Average amount of time it takes the read head to find the right track and access the data. In other words, it represents the average length of time it takes the disk to provide data after having received the order to do so. It must be as short as possible.
• Radial density: number of tracks per inch (tpi).
• Linear density: number of bits per inch (bpi) on a given track.
• Surface density: ratio between the linear density and radial density (expressed in bits per square inch).
• Cache memory (or buffer memory): Amound of memory located on the hard drive. Cache memory is used to store the drive's most frequently-accessed data, in order to improve overall performance; 
  
• Interface: This refers to the connections used by the hard drive. The main hard drive interfaces are:
  • IDE/ATA
    
  • SERIAL ATA
    
  • SCSI
    
  • However, there are external cases used for connecting hard drives with USB or FIRE WIRE ports.