A modern hard disc consists of following basic parts: a printed circuit board, a hermetical unit and a software part.
The hermetical unit serves for protection and fastening of a magnetic discs package - data carriers and magnetic-head assembly (MHA) and its mode of functioning is in use of materials changing their electric resistance at change of the surrounding magnetic field intensity. Magnetic discs are covered with a magnetosensible layer which is a data carrier. The information on it is in the form of magnetic fields created by the smallest sites of evaporated layer. The reading head, being put in a field created by the magnetic coating site, forms an electric signal as it is a conductor located in the magnetic field. This signal is processed in appropriate way and transferred further to the interface.
On some hard discs there are check boxes for a servo writer bar (device for recording servo labels on the store plates), covered with a strong foil-clad material. The hermetical unit is filled with dust-free air which is communicating with environment through special compensating apertures closed by filters, for pressure equalizing inside the hermetical unit.
As the figure shows, the heads are fastened to the metal rocker, which is mobile relative to magnetic discs. During the engine running rotating magnetic discs create the air stream which forms "an air cushion" between the head and the surface. When the spindle engine stops, the rocker is automatically drawn aside (as a rule, under pressure of the air stream from still rotating discs) to a so-called parking position in which it is fixed by a special lock or a magnetic latch. In this position the run-in heads are drawn away of the working zone, preventing any possible contact between the rocker and the disc surface, thus protecting a vulnerable working surface. The park zone is located, as a rule, closer to the spindle axis. But there are also exceptions, in particular, in the storage discs of portable computers the park zone is located on special parking racks outside the plates.
In modern storage discs the head positioning actuator (a device, positioning the reading/recording head above the disc) moves together with a rocker along magnetic discs with the electromagnet. In the tail part of the head positioning actuator there is a coil covered from above and from below with magnetic plates, rigidly fixed on the hermetical unit which serve as stator. The current passing through the coil winding makes the head positioning actuator deviate with certain acceleration, and its deviation direction can be changed by the current direction change in the actuator winding. Such a control layout is called Voice Coil.
There is a preamplification/commutation chip inside the hermetical unit. Such a strange, at first glance, location (inside the hermetical unit, instead of on the printed-circuit-board, close to all other electronic components) is explained very simply: the preamplifier should be placed as close to reading/recording head as possible for the path between the head and the preamplifier cut-back and noise reduction. The preamplifier signal goes over the multicore cable to the printed-circuit-board where it is transformed for IDE controller of the computer system board.
In real operation the storage disc should read data from the surface. For this purpose it should recognize the head position at the moment and where it is to be moved. To this end there is servoinformation on the disc surface, unique for each data track and sector.
The printed circuit board consists of following basic components: a central processor, operative memory module (RAM), a ROM chip (in modern stores ROM is integrated into the processor, a so-called shadow-mask ROM), a spindle engine and MHA control chip, a power socket and an interface socket.
We should pay a special attention to the store microprogram. As a rule, it consists of two parts: the initial loader and the basic program (overlay, translator with defect - sheets, SMART modules, factory calibration and self-testing modules, logs and so on). Basically, the loader is located on the printed circuit board as a processor together with a ROM. Having started the printed circuit board and accelerated the engine up to the necessary speed, the microprogram positions the head according to servo labels and tries to read the service information. If everything goes without failures, the disc gets recalibrated and is in readiness step on the interface.
Certainly, the given article is of general character so it cannot capture all nuances and subtleties of structure of a modern hard disc which is, no doubtl, a rather complex device, as a matter of fact, reminding a miniature computer .
The information on current problems on hard discs repair is regularly published in the section of articles where many aspects of some models hard disc internal structure are considered.