Modern Hard disk drive(一)

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    Brief architecture description, the main problems of modern hard disk drives, methods of HDD servicing and repair of simple malfunctions, SMART, passwords. The article is intended for data recovery specialists, technicians servicing computer equipment, network administrators and experienced users.

Drive construction
    A drive consists of a mechanical part - head-and-disk assembly (HDA) and a printed circuit board (PCB). HDA acts as a case for all mechanical parts of a hard drive and contains one more chip performing the functions of a preamplifier/commutator. A PCB consists of several chips which control the mechanical parts, encode/decode data on magnetic surfaces and transfer the data through an external interface. PCBs are located outside HDA, in its lower part as a rule. In some hard drives, like the well-known seagate Barracuda series, the controller has an additional metal cover protecting the electronic components from damage.

Mechanics
    The whole construction is based on the drive case protecting sensitive mechanical parts from environmental influence. Inside it is filled with dust-free air though the air is not specifically purified; instead the assembly of the mechanical part is performed in a special workshop where air contains less than one hundred dust particles per cubic meter, i.e. in the so-called "class 100 clean room".
HDA case has an opening blocked by a tight air filter. It is used to align air pressure inside the HDD and outside. Unfortunately, if a drive falls into water, the latter penetrates the inner space through that opening.  Rotation of disks creates air flow circulating inside the case and constantly passes through one more filter separating dust if it somehow appears inside.
     Drive case accommodates a pack of magnetic disks driven by a spindle motor, magnetic heads with their positioning system and a preamplifier/commutator enhancing the signal from the heads and switching between them.
A magnetic disk is a circular aluminum (rarely ceramic or made of special glass) plate with surface polished in accordance with the highest  precision class for the sole exception of the parking zone, if it is present. In fact, high precision of disk surfaces and the heads causes them to "stick" to each other because of molecular attraction forces. To prevent that effect, manufacturers use special laser serrations in the zone of contact between drive heads and disks.

    The disks demonstrate specific magnetic properties owing to their chrome oxide based coating (magnetically active substance) or cobalt layer applied using vacuum deposition. Such coating is characterized by high hardness and much greater wear resistance compared to previous models coated with a layer of soft varnish based on ferric oxides which could be easily damaged unlike modern coatings.

    The disks are rotated by a special 3-phase electric motor. The stationary part contains three windings connected according to the "star" scheme, with a tap in the middle, and the rotating part is a permanent sectional magnet made of rare-earth metals. The requirement of beat reduction and high rotational speed values force the manufacturers to use special bearings in the spindle motor; these can be either ball bearings or improved fluid bearings (using special oil dampening impact loads and thus increasing motor durability). Fluid bearings are characterized by a lower noise level and produce practically no heat during operation. The number of revolutions per minute in modern IDE drives is equal to 5400 RPM or 7200 RPM; for modern SCSI drives it is 10000 RPM or 15000 RPM.

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