All computer data must be physically recorded somewhere right? The hard disk (HDD) is the organ of the computer used to keep the data permanently, unlike the RAM, which disappears every time the computer is restarted.

This is why it is sometimes referred to as mass memory to designate hard drives. The hard drive is connected to the motherboard via a hard disk controller that interfaces between the processor and the hard drive. The hard disk controller manages the disks that are connected to it, interprets the commands sent by the processor and routes them to the disk concerned.

USB hard drive

With the introduction of the USB standard, external boxes allowing to connect a hard disk to a USB port have appeared, making the hard disk easy to install and allowing to add storage capacity to make backups.

This is referred to as an external hard drive as opposed to internal hard disks connected directly to the motherboard, but they are the same disks except that they are connected to the computer via an external hard drive, a box connected to a USB port.

  • Structure

A hard disk is made of not one disc, but several hard disks made of metal, glass or ceramic, stacked at a very short distance from each other and called Platters.  The disks rotate very rapidly around an axis (at several thousand revolutions per minute currently) in a counterclockwise direction. A computer operates binary, that is, the data is stored as 0 and 1 (called bits).

Reading and writing is done by heads located on either side of each of the trays. These heads are electromagnets that lower and rise in order to read the information or write it. Moreover, these heads are movable laterally in order to be able to sweep the entire surface of the disk. 

  • Operation

The read / write heads are said to be “inductive”, that is to say that they are capable of generating a magnetic field. This is particularly the case during writing: the heads, by creating positive or negative fields, polarize the surface of the disk in a very small area, which will translate during the passage through reading by changes in polarity inducing A current in the read head, which is then transformed by an analog-digital converter (CAN) into 0 and into 1 understandable by the computer.

  • Cylinder

A cylinder is the set of data located on the same track on different trays (that is, placed vertically above each other) because it forms a “cylinder” of data in space. Finally, we call cluster (or in French allocation unit ) the minimum area that a file can occupy on the disk. Indeed, the operating system exploits blocks which are in fact several sectors (between 1 and 16 sectors). 

  • Block Mode

Block mode and 32-bit transfer allow you to fully exploit the performance of your hard drive. Block mode consists in carrying out data transfers per block that is in packets of 512 bytes generally, which prevents the processor from having to process a multitude of tiny packets of one bit. The processor then has “time” to perform other operations.

Unfortunately, this data transfer mode is only useful for older operating systems, because recent operating systems use their own hard disk drive, which makes this manager obsolete.

  • 32-bit mode

 The 32-bit mode (as opposed to the 16-bit mode) is characterized by 32-bit data transfer. The 32-bit transfer corresponds to 32 doors that open and close simultaneously. In 32-bit mode, two 16-bit words (set of bits) are transmitted successively and then assembled. The performance gain associated with switching from 16-bit to 32-bit is generally insignificant. The solution in this case is to connect a 32-bit hard disk to the same tablet as the CD-ROM drive.

 Technical characteristics:

  • Internal

By default, the hard drive is internal, that is, integrated into the computer and directly connected to the motherboard. Unless changed by the user, it is on this disk that the operating system starts. An internal hard drive is usually plugged in using an S-ATA port on the motherboard.

  • External

A hard disk can be external, i.e. it is an additional hard drive connected to one of the computer’s input / output ports, usually the USB port. Technically, an external hard drive is usually an internal (internal) hard drive placed in a case enabling it to be plugged into one of the motherboard’s I / O ports.

  • Capacity

The amount of data that can be stored on the disk.

  • Transfer rate (throughput)

The amount of data that can be read or written to disk per unit of time. It is expressed in bits per second.

  • Rotation speed

The speed at which the plates rotate, expressed in revolutions per minute (denoted rpm for rotations per minute). The speed of the hard drives is in the order of 7200 to 15000 rpm. The higher the rotational speed of a disk, the better the disk rate. On the other hand, a disk having a high rotational speed is generally noisier and more easily heated.

  • Latency

Also called rotational delay: time elapsed between the time the disc finds the track and the time it finds the data.

  • Average access time

Average time that the head put to position on the right track and access the data. It therefore represents the average time that the disk takes from the time it is ordered to provide data to the time it actually provides it. It must therefore be as short as possible.

  • Radial Density

Number of tracks per inch (TPI: Track per Inch). 

  • Linear density

Number of bits per inch on a given track (BPI: Bit per Inch).

  • Surface density

Ratio of linear density to radial density (expressed in bits per square inch).

  • Cache memory (buffer memory)

The amount of memory on the hard disk. The cache memory keeps the data that the disk accesses most often in order to improve overall performance.