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A Hard Disk Drive Under The Microscope

Updated on October 14, 2014

Main Causes of Hard Drive Failure: Our Experience

Users often try to handle the situation themselves. This qualified user's desire to restore information on his own can be understood. But about a half of data loss cases is associated with physical damage!

Some "professionals" try to repair their HDDs at home by themselves. They don't confess. But where do fingerprints on the mirror surface of hard drive platters come from? You think, a robot hasn't washed his hands before assembling a device on the assembly line?

Physically damaged elements are in the HDD box? In this case nothing can be done. A head crashed? Our specialists can help you then. They will unseal the box, mount a new head and read lost data. However, only HDD data recovery is possible in this case. Further HDD use is not.

The term "hard drive", or more precisely "hard disk drive", is known to everybody. The operating system is stored there. It is also loaded into the computer's memory from the HDD.

A completely sealed HDD metal box looks reliable and durable. Such an impression may be the reason for rough handling. Data loss and device failures are inevitable then.

To understand possible reasons for HDD failures let's become familiar with the device construction and principles of its operation.

What Is Inside?

HDD operation is based upon the principle of magnetic signal writing/reading to/from the rotating disc. The disk is covered with a magnetically susceptible working layer. Each side of the disc covered with the working layer is called working surface.

When writing digital data is converted into analog electrical signals. They create sections with different values of magnetization with the help of the writing head. The sections are located along the circumference of the entire working surface of the rotating disc. They are called tracks. Section size and the distance between adjacent tracks define the surface density of data recording.

When reading disc sessions are moving under the magnetic head. They induce electrical signals in it. These signals are then converted into digital data.

A typical modern hard disc drive is composed of a disc pack, a speed spindle motor, a block of heads for writing/reading, a pre-switch device and a controller. The latter is a printed circuit board with control electronics.

While in an idle mode, the head is pressed against the disc surface by the work carrier. This happens in a special non-working area that is called the landing zone. The first HDD models required a special head landing operation. It was initiated by software.

In modern HDDs the operation of placing heads in the landing zone is performed automatically. This happens when the motor rotation speed is below the nominal speed. Or when the power supply fails. Head placing in the working area is possible only at a nominal rotation disc speed. The gap between the head and the disc surface is a few hundredth of a micron.

The Head Is An Important HDD Component

The maximum areal density is to a large extent defined by the construction and head characteristics. In the past different heads were mounted in hard drives. They were miniature inductance coils. These coils were wounded on the magnetic core.

Later engineers began to use thin-film magnetic heads. Today high-sensitivity magnetoresistive read heads are used. The principle of their operation is based on the effect of anisotropy of some semiconductors in the magnetic field. These heads are structurally combined with thin-film recording heads. Heads are assembled in a block.

In today's hard drives a positioning system of the block of heads with a rotary movable coil is used. The coil is placed in the gap of a powerful permanent magnet. The coil is the actuator of the positioning system.

At the heart of this system is preliminary recording of special digital sequences. They are called servo marks. Servo marks are recorded to appropriate sectors of every track. During operation the controller is guided by these marks. It generates control signals. They go to a movable coil. The head then rotates so that it is fixed directly over the track. The head remains in these position over the track until the command to change head position comes.

Hard Drive Aerodynamics

To increase the recording density the gap between the HDD surface and the head should be reduced to a minimum. In today's hard drives this problem is solved by using aerodynamic lift force. It is generated by the air flow. Rotating disc surface carries the air flow along. Head working surfaces are in the form of the wing. This is done to create lift force. The head is fixed by a springy work carrier. Therefore, it doesn't "fly away" far from the disc surface.

The magnitude of lift force is defined by the air density. The air density is by-turn dependent on the atmospheric pressure. Therefore, general hard drives have maximal lift height-above-sea-level limitations. They are about 2000-3000 m.

In today's HDDs the rotation speed of the disc pack may reach 15,000 r/min. But high speed causes some problems. They are associated with balancing, gyroscopic effect and head aerodynamics. During operation, heads by no means should be in a physical contact with working surfaces. An accidental contact almost always leads to complete or partial damage of a corresponding track. This also very often causes a head crash.

Clean Means Healthy

A disc pack, an engine and a head block are located in a sealed box - an HDA. It is equipped with a removable lid. The HDA internal volume is not isolated from the external environment. There is always a possibility of letting air flow into the HDA and vice versa. It is necessary to equalize the pressure inside the HDA to the external pressure to prevent deformations of the case.

The problem is solved by using a so-called barometric filter. It is placed inside the HDA. The filter is capable of retaining certain particles. Their size must exceed the working gap between the head and the working surface. Moreover, they can damage the working surface and the head if in the working gap. Thus, case hermeticity is nothing but the inability of such particles to get inside. Moreover, there is always a recycling filter in the HDA. It is designed to trap particles that can spring up inside the HDA. This happens because of the process of disc surface shedding as heads "take off" and "land" in the landing zone. Some particles can also get into the case through the barometric filter. Recycling filter location is chosen in consideration of air flow direction so that the maximum purification efficiency of the air flow inside the HAD is provided.

What Operates Can Break Down

Such a complex structure of the hard drive has many bottlenecks. They can cause different failures. In general, they are associated with natural wear of mechanical blocks and the aging process of electronic components. As a rule, guaranteed average Mean Time Between Failures (MTBF) is 0.5-1 million hours. This means that the hard drive operates permanently during 57-114 years. But this characteristic is rather theoretical than practical.

The magnetization of the working layer is gradually waning over time. The recording quality of user's files remains almost the same. Files are usually rewritten. But servo marks, control sector marks and data in the engineering area are recorded by the manufacturer once and for all. Therefore, some time after there may emerge data access problems. They are caused by overhead magnetic damping.

You can have the engineering area rewritten by data retrieval professionals. The same applies to the defect matrix. It can be restored without losing user's data. However, restoring servo marks is impossible without destroying data even by the manufacturer's effort.

Not less than 16% of the total working surface is allotted to overhead information. It provides normal operation of the hard drive. First of all, it's the engineering area (configuration sectors, defect matrices, executable hard drive code).

Remaining disc space is divided into zones. Most hard drives have 8-20 zones. Each zone contains a different number of sectors. Not all sectors are used as working ones. Some sectors are reserve.

Initial disc formatting at the manufacturer's plant involves disc surface checkup. The information on found defective spots is recorded to the defect matrix. It is located in the engineering area.

During hard drive operation this matrix is used for reassigning of bad access sectors to good access sectors. The latter sectors are located in the reserve tracks. Overhead information is important. Therefore, the engineering area of different HDD models may contain 2-6 copies. Servo marks include reserve ones. They are recorded by a more powerful magnetic field.

During operation time magnetic disc surface is gradually destroyed. New bad sectors begin to appear. This leads to so-called magnetic medium degeneration. Regular write and read error indicate such problems.

Spindle motor bearings may wear out or be damaged. This causes platters to swing a little bit. The distance between heads and working surfaces alternately increases and decreases. This is the reason for the increase in the number of "mild" errors and a high risk of head falling on the hard drive surface.

Did your hard drive begin to produce a strange noise? Or can you hear the mechanism of head positioning clicking against stoppers? Get ready for avalanche-like hard drive failures soon.

In such cases information can often be saved. But hard drive repair works and its further use are impossible.

Power Supply Failures

Instability of supply voltage may result in the failure of the hard disc controller electronics. A magnetic “blow” can also be the result. It affects the hard drive working surface. Hard drive controller burn-in cases are quite frequent. They can lead to severe consequences. The electronic auto-landing mechanism fails to work. As a result, heads land on the disc surface in the working area (not in the landing zone). This leads to damage of not only working surfaces but heads too.

Voltage oscillations are another reason for data loss and hard drive failures. These failures can be prevented by the user. The user can buy an uninterruptible power supply (UPS) that stabilizes voltage.

One More Is Down

The frequent reason for hard drive failures is temperature regime violation. The rotation speed of the disc pack reaches 15,000 r/min. To improve dynamic characteristics powerful spindle motors and positioning drives are used. They generate a significant amount of heat. In addition to that, a mounting bearing also generates heat.

A hard drive is often located close to other devices in the computer casing. It is not cooled enough. And this leads to its overheating. This may result in the severest consequences. Among them are spindle motor jamming (plus a simultaneous failure of power components of the controller), damage of the hard drive working layer and heads sticking to the surface in the landing zone.


A major group of factors that lead to hard drive failures and data loss is associated with incorrect users’ actions. For example, shocks and vibrations disturb balancing and disc alignment. That doesn’t necessarily lead to performance loss. Poor balancing results in vibrations, fast wear of mounting bearings, HDA overheating and, finally, a premature failure of the system.

In a disabled state radial shocks are especially dangerous for the HDD. As a rule, these shocks result in complete hard drive performance loss and impossibility of its further use. In some cases data recovery from such a medium can be performed. But its further operation is impossible.

A medium in a working state is susceptible to both radial and axial mechanical impacts. The latter may occur for such an insignificant reason as an accidental push of the computer casing and others.

Heads “fly” over the disc surface at a height of only 0.1-0.12 microns. Even a weak stroke is enough to reduce this distance to a zero. The track moves under the magnetic head at a speed of 90-125 km/h. Can you imagine the force of impact in this collision? 1 mm of a track is equal to 2 Kbytes. Do you see the loss?

Let’s assume that the damage is insignificant. Knocked-out magnetic particles will be flying inside the case for a long time. This will create the risk of another failure. Much of particles remain stuck to the disc surface. This leads to the further process of destroying the working surface.

In addition to that, when in direct contact with the disc surface the head overheats in an instant. As a result of this head crash and along with a radial mechanical impact, the head breaks.

Without a Spacesuit

The same fatal consequences may be caused by depressurization. Hermetic sealing of some hard drive models (Western Digital, Seagate, Quantum) is provided by adhesive tape. It is stuck down along the perimeter of the hard drive casing. Such hard drives require a good care when storing, transporting and mounting the HDD in the computer casing. Installation of such hard discs into some computer casings is fraught with unintended depressurization.

Some hard drive models have operational openings. They are covered with special stickers. A user may damage them if he handles the medium roughly.

If the HDA is depressurized, even dust may lead to fatal consequences. They include gradual destruction of the working layer and head crashes. Discs with depressurized casings can’t operate properly.

Connect Properly

Wrong connection of the power cables almost always result in a complete breakdown of hard drive electronic components. The pre-switch device in the hard drive casing can also be damaged for this reason.

Users often improperly connect interface cables. Consequences of this operation are difficult to predict. Sometimes it doesn’t lead to fatal consequences for the medium and the computer motherboard. Sometimes the breakdown of the hard drive controller or the motherboard interface occurs.

Preventive Measures

In most modern hard drives such technology as SMART (Self-Monitoring Analysis and Reporting Technology) is implemented. Its essence lies in the fact that the hard drive performs self-diagnostic check. The hard drive can warn the user of the pre-emergency condition in advance. Most SMART HDDs exercise control over 2-30 attributes of reliability. These include the number of facts of head positioning, the number of positioning errors and so on.

We recommend you to regularly undertake a number of simple and effective measures to prevent data loss. They are widely known. But users often neglect them.

These measures include regular backups. In addition to that, it is helpful to regularly use diagnostic software and defragmentation programs (like Norton Utilities).

Regular diagnostics will help you to notice gradual degradation of your HDD in a proper time. The symptoms include decrease in performance and bad sectors. In this case you'd better copy all necessary information to another medium. And Data Retrieval will perform diagnostics of your device. Our company has all necessary equipment to tell you whether you can use your HDD further or not.

So if you have any problem with hard disk, contact Info Recovery LLC. We will save your time and money.

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      5 years ago from Here

      I have learned a lot from this awesome lens!!! Thanks a lot for sharing this great topic!

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      6 years ago

      What a great informative lens on Hard disk drive! :)

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      6 years ago from Abbotsford, BC

      I appreciate your computer repair tutorials, many thanks


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