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    2021-10-21 17:41:16
    What is a Hard Disk Drive (HDD)?


    Ⅰ Hard Disk Drive Technical Parameters
    Ⅱ Hard Disk Drive Structure  1.Read-and-write head
    Ⅲ Things you need to know

    Ⅰ Hard Disk Drive Technical Parameters

    1. Capacity

    Hard Disk Drive.jpg

    Hard Disk Drive

    As the data storage of a computer system, capacity is the most important parameter of a hard disk. The capacity of the hard disk drive is in megabytes (MB) or gigabytes (GB), 1GB=1024MB, 1TB=1024GB. However, the hard disk drive manufacturer usually takes 1G=1000MB in the nominal hard disk capacity, so the capacity we see in the BIOS or when formatting the hard disk will be smaller than the manufacturer’s nominal value.

    The capacity index of the hard disk drive also includes the single disk capacity of the hard disk. The so-called single disk capacity refers to the capacity of a single disk of a hard disk drive. The larger the single disk capacity, the lower the unit cost and the shorter the average access time.

    2. Rotational Speed

    Rotational Speed or Spindle speed is the rotation speed of the motor spindle in the hard disk drive, which is the maximum number of revolutions that the hard disk platter can complete in one minute.

    The faster the rotation speed of the hard disk drive, the faster the hard disk drive can find files, and the transmission speed of the relative hard disk has also been improved. The hard disk drive speed is expressed in revolutions per minute, and the unit is expressed as RPM. RPM is the abbreviation of Revolutions Per minute, which is revolutions per minute. The larger the RPM value, the faster the internal transfer rate, the shorter the access time, and the better the overall performance of the hard disk drive.

    3. Average Access Time

    The average access time refers to the time required for the head to reach the target track position from the starting position and find the data sector to be read and written on the target track.

    The average access time reflects the read and write speed of the hard disk, which includes the seek time and waiting time of the hard disk drive, namely: average access time = average seek time + average waiting time.

    The average seek time of the hard disk drive refers to the time required for the head of the hard disk to move to the specified track on the disk surface. This time is of course as small as possible. The average seek time of a hard disk is usually between 8ms and 12ms, while a SCSI hard disk should be less than or equal to 8ms.

    The waiting time of the hard disk drive, also known as the latency, refers to the time when the head is already in the track to be accessed and waiting for the sector to be accessed to rotate below the head. The average waiting time is half of the time required for the disc to rotate one round, and should generally be less than 4ms.

    4. Transmission rate

    The data transfer rate of the hard disk drive refers to the speed at which the hard disk reads and writes data, in megabytes per second (MB/s). Hard disk data transfer rate includes internal data transfer rate and external data transfer rate.

    The internal transfer rate is also called the sustained transfer rate, which reflects the performance of the hard disk buffer when it is not in use. The internal transfer rate mainly depends on the rotation speed of the hard disk.

    External transfer rate is also called Burst Data Transfer Rate or interface transfer rate. The maximum external transfer rate of the Fast ATA interface hard disk is 16.6MB/s, while the Ultra ATA interface hard disk reaches 33.3MB/s.

    5. Cache

    Cache memory is a memory chip on the hard disk controller with extremely fast access speed. It is a buffer between the internal storage of the hard disk drive and the external interface. Since the internal data transfer speed of the hard disk drive is different from the transfer speed of the external interface, the cache plays a role as a buffer among them. The size and speed of the cache is an important factor directly related to the transmission speed of the hard disk, which can greatly improve the overall performance of the hard disk drive.

    Ⅱ Hard Disk Drive Structure 

    The hard disk drive is one of the most important storage in a computer. Most of the software needed for the computer to function properly is stored on the hard drive. Because the storage capacity of hard disks is large, it is different from memory and optical disk. Hard disk drives are storage devices based on hard rotating disks used on computers. It stores and retrieves digital data on a flat magnetic surface.

    1.Read-and-write head

    Hard Disk Structure 1.png

    Hard Disk Structure (1)

    The read-and-write head is the most expensive part of the hard disk, and it is also the most important and critical part of the hard disk drive technology. The traditional magnetic head is an electromagnetic induction magnetic head that combines reading and writing. However, hard disk reading and writing are two completely different operations. For this reason, this two-in-one magnetic head must be designed with both reading and writing. These two characteristics result in the limitation of hard disk design. The MR magnetic heads, that is, magnetoresistive magnetic heads, use a separate head structure: the write head still uses the traditional magnetic induction head (MR head cannot write), and the read head uses a new type of MR head. The so-called inductive writing and magnetoresistive reading. In this way, during the design, the different characteristics of the two can be optimized separately to obtain the best read/write performance.


    When the disk rotates, if the head is held in one position, each head will draw a circular track on the surface of the disk. These tracks are invisible to the naked eye at all, because they are only some magnetized areas on the disk surface magnetized in a special way, and the information on the disk is stored along such tracks. Adjacent tracks are not close to each other. This is because when the magnetization units are too close, the magnetism will affect each other, and it will also bring difficulties to the reading and writing of the magnetic head. A 1.44MB 3.5-inch floppy disk has 80 tracks on one side, and the track density on the hard disk is much greater than this value, usually, there are thousands of tracks on one side.


    Hard Disk Structure 2.png

    Hard Disk Structure (2)

    Each track on the disk is equally divided into several arc segments. These arc segments are the sectors of the disk. Each sector can store 512 bytes of information. The disk drive reads and writes data to the disk and the unit sectors. Each track of 1.44MB 3.5-inch floppy disk is divided into 18 sectors.


    A hard disk drive is usually composed of a set of overlapping disks. Each disk surface is divided into an equal number of tracks and numbered from the "0" on the outer edge. The tracks with the same number form a cylinder, which is called the cylinder of the disk. The number of cylinders on a disk is equal to the number of tracks on a disk. Since each disk surface has its own head, the number of disk surfaces is equal to the total number of heads. The so-called CHS of the hard disk, namely Cylinder, Head, Sector, as long as the number of CHS of the hard disk is known, the capacity of the hard disk can be determined, and the capacity of the hard disk = the number of cylinders and the number of sectors. The number is 512B.

    Ⅲ Things you need to know

    1. Don't shut down the computer suddenly while working

    When the hard disk starts to work, it is generally spinning at a high speed. If we suddenly turn off the power, it may cause violent friction between the head and the platters and damage the hard disk. Therefore, it is necessary to avoid a sudden shutdown. When shutting down, you must pay attention to whether the hard disk indicator on the panel is still flashing. Only after the indicator stops flashing and the hard disk read and write ends, you can turn off the computer's power switch.

    2. Prevent dust from entering

    Dust can cause great damage to the hard disk drive. This is because, in a severely dusty environment, the hard disk can easily attract dust particles in the air, causing them to accumulate on the internal circuit components of the hard disk for a long time, which will affect the heat of the electronic components.

    In addition, dust may also absorb moisture, corrode the electronic circuits inside the hard disk drive. Therefore, although the size of dust is small, the harm to the hard disk cannot be underestimated. Therefore, it is necessary to maintain environmental sanitation and reduce the humidity and dust content in the air. Remember: general computer users cannot disassemble the hard disk cover by themselves, otherwise the dust in the air will enter the hard disk and scratch the disk or the magnetic head when the magnetic head reads or writes.

    3. Prevent the temperature from being too high or too low

    Temperature also affects the life of the hard disk drive. A certain amount of heat is generated when the hard disk is working, and there is a heat dissipation problem during use. The temperature should be 20~25℃, too high or too low will cause the clock frequency of the crystal oscillator to change. Temperature can also cause the failure of hard disk circuit components, and magnetic media can also cause recording errors due to thermal expansion. In addition, try not to place the hard disk close to strong magnetic fields, such as speakers, motors, radios, mobile phones, etc., to avoid damage to the data recorded on the hard disk due to magnetization.

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