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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.
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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.
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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.
A magnetic head is also a sophisticated construction composed of numerous details. Those details are so small that they are manufactured using photolithography method just like chips. Working surface of the head's ceramic case is polished with the same precision as the disk itself. Heads' actuator is a flat solenoid coil of copper wire suspended between the poles of a permanent magnet and fixed at one end of a lever rotating around a bearing. The other end of the lever is connected to a bracket carrying magnetic heads. The bracket is spring-loaded with a certain effort which allows the heads to "fly" at a definite height above the disk surface; the said height is usually equal to tenths of micron.
The whole transport system moving the heads' pack has been called Voice Coil by analogy with a loud-speaker cone. Its functional principle is similar to that of a common dynamic loud-speaker (i.e. copper coil in static magnetic field). Positioner's coil is surrounded by a stator acting as a permanent magnet. When electric current of certain voltage and polarity appears in the coil the positioner starts turning to the corresponding side with respective acceleration; thus dynamic modification of current properties in coil allows positioning of magnetic heads to any location above disk surface.
Drive heads are fixed when a drive is powered-off (in the parking zone) with special latches. Magnetic and pneumatic latches are two most widely used types. A magnetic latch is a small permanent magnet fixed within drive case and attracting ferrous lug on the voice coil in the heads' parking position. Pneumatic latch (or air lock) also fixes a positioner in the parking zone preventing its further movement. When the magnetic disks begin rotation the air flow thus generated deflects the "sail" of an air latch and unblocks the positioning system.
The electronic components inside HDA are limited to the preamplifier/commutator for the signal received from drive heads. It is located closer to the heads to minimize interference of external noise, right over the flexible cable from the heads to drive's electronics. The same cable is connected to the voice coil and, sometimes, to the spindle motor; however, in most cases power supply of the spindle motor is implemented via a separate cable.
A HDA is usually linked to the PCB with two connectors. One of them is a three-phase center-tapped connector for the spindle motor while the other delivers signals from the preamplifier/commutator and voice coil.
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The circuit design of modern drives is characterized by the use of a few highly-integrated chips; their block diagram is represented in figure 1.
Figure1. Circuit design of modern drives
As one can see in the picture, the whole layout is based upon four chips:
- system controller chip including the read/write channel, disk controller and RISC control processor (microcontroller);
- Flash ROM chip containing drive firmware;
- chip controlling the spindle motor and voice coil;
- ROM chip used as a cache buffer.
Further increase of integration is impossible due to some basic differences in the operational modes of the above functional parts.
The first system controller used in hard drives was a chip manufactured by Cirrus Logic. Its obvious breakthrough was manifested in the read/write channel, processor and disk controller integrated within one chip; however insufficiently developed methods of using such a microcircuit caused frequent malfunctions of Fujitsu drives belonging to series MPF3xxxAT and MPG.
A microcontroller has RISC architecture. As soon as power supply is switched on after the /RESET interface signal the drive reset circuit sends a RESET signal to microcontroller which executes its program from ROM running self-diagnostics, cleaning the working data area in memory and programming disk controller and all programmable chips connected to the internal data bus of an HDD. Then microcontroller polls internal signals used during drive operation and if it detects no emergency alerts, it starts the spindle motor. The next stage of firmware operation is internal testing of an HDD checking data buffer RAM, disk microcontroller and the status of microcontroller signals input from its port. Then the microcontroller begins analyzing the frequency of pulses waiting until the spindle motor reaches defined rotational speed. As soon as the necessary speed is reached, the controller begins to manipulate the positioning circuit and disk controller moving the magnetic heads to the area containing recorded firmware data and transfers it to buffer RAM for further operation. Then the microcontroller switches to readiness and awaits commands from HOST. In that mode a command received from the central processor initiates a whole chain of actions performed by all the electronic components in a HDD.
HDD read/write channel consists of a preamplifier/commutator (located inside HDA), read circuit, write circuit and a synchronizing clock.
Drive preamplifier has several channels, each being connected to its respective head. The channels are switched by signals from the drive’s microprocessor. Preamplifier also contains a recording current switch and recording error sensor, which emits an error signal if a short circuit or break occurs in a magnetic head.
Integrated reading/writing channel operating in the recording mode receives data from disk controller simultaneously with the recording clock frequency, performs data encoding, precompensation and transfers the data to preamplifier for writing to a disk. In the reading mode signal from preamplifier/commutator is transmitted to the automatic control circuit and then passes a programmable filter, adaptive compensatory circuit and pulse detector while being converted into data pulses sent to the disk controller for decoding and transfer through an external interface.
Disk controller is the most complicated drive component which determines the speed of data exchange between a HDD and HOST.
Disk controller has four ports used for connection to a HOST, microcontroller, buffer RAM and data exchange channel between it and HDD. Disk controller is an automatic device driven by microcontroller; from HOST side only standard registers of task file are accessible. Disk controller is programmed at the initialization stage by microcontroller, during the procedure it sets up the data encoding methods, selects the polynomial method of error correction, defines flexible or hard partitioning into sectors, etc.
Buffer manager is a functional part of disk controller governing the operations of buffer RAM. The capacity of the latter ranges in modern HDDs from 512 Kb to 8 Mb. Buffer manager splits the whole buffer RAM into separate sectioned buffers. Special registers accessible from microcontroller contain the initial addresses of those sectioned buffers. When HOST exchanges data with one of the buffers the read/write channel can exchange data with another buffer sector. Thus the system achieves multisequencing for the processes of data reading/writing from/to disk and data exchange with HOST.
Spindle motor controller regulates the motion of a 3-phase motor. It is programmed by the drive microcontroller. There are three control modes of spindle motor operation: the start mode, acceleration mode and stable rotation mode. Let us review the start mode. At power-up a reset signal is sent to the control microprocessor which performs initialization programming internal registers of spindle motor controller for a start. Drive controller generates phase switching signals; the spindle motor at that rotates at low speed generating self-induced electromotive force. Drive controller detects EMF and notifies the microprocessor which uses that signal for rotation control. In the acceleration mode microprocessor speeds-up phase switching and measures the rotational speed of the spindle motor until the speed reaches its rated value. As soon as the rated rotational speed is reached the controller introduces stable rotational mode. In that mode microprocessor calculates the time required for one revolution of the spindle motor based on the phase signal and adjusts the rotational speed accordingly. After relocation of magnetic heads from the parking zone the drive electronics begins tracking the stability of rotation using servo marks.
Voice coil controller generates the control current moving drive positioner and stabilizing it over a defined track. Current value is calculated by microcontroller on the basis of digital error signal for head position relatively to a track (Position Error Signal or PES). Current value in digital form is transmitted to CPU, the analogous signal thus received is enhanced and supplied to the voice coil.
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| Firmware data (service information) |
Firmware data is necessary for functioning of internal HDD circuits and as a rule it remains hidden from users. Firmware data can be subdivided into the following types:
- Servo information or servo fields;
- Low-level format;
- Resident firmware microcode (operational programs);
- Configuration tables and settings;
- Tables of defects.
Servo fields are necessary for operation of a servo system used by the driving assembly of magnetic heads in a HDD; they serve for heads’ positioning and keeping them precisely over a defined track. Servo fields are recorded during the manufacturing process to an already assembled HDA through special service openings in its case. The openings are subsequently closed with sticky labels that read: Warning! DO NOT OPEN. The recording is actually performed using drive’s own heads in a special high-precision instrument – servo writer. Relocation of heads’ positioner is achieved through a motion of a special pusher of the servo writer using steady steps much smaller than the intervals between tracks.
Firmware (microcode) of the control microprocessor is a collection of programs required for operation of HDD components. Here belong the programs used for initial diagnostics, control of spindle motor rotation, data exchange with disk controller, buffer RAM, etc. In most HDD models firmware microcode is stored within internal microcontroller ROM; some models employ external Flash ROM. In some HDD models a part of firmware programs is recorded to magnetic disk in a special firmware zone while ROM contains the programs used for initialization, and positioning together with primary loader reading the firmware data from magnetic disk to RAM. Since actual firmware modules are first loaded to RAM before execution they have been called resident modules.
Manufacturers of hard drives record some firmware portions on disk surface not only for purposes of ROM space saving, but also to enable its easy replacement if the manufacturing process or drive operation reveal any errors in a microcode. Internet pages of most manufacturers contain links to utilities used for such updates. Overwriting disk firmware is much easier than unsoldering of hard-programmed microcontrollers. We can remember how Western Digital had to recall a large number of its drives back to factory several years ago…
Low-level format. Track beginning is identified by an index pulse. Each track is subdivided into data sectors and servo fields. Format of each sector consists of an ID field, data field, synchronization zones and spaces. The beginning of each sector contains a synchronization zone used for phasing and synchronization of data strobe. ID field contains an address marker, physical sector address, flag byte and CRC bytes.
Format without identifiers has become popular recently. When manufacturers employ such method of data placement along a track ID fields are not used at all (thus increasing available drive capacity). Instead they use a system of servo fields directing to physical sectors on a track. At that reading/writing of all sectors on a track is performed simultaneously (in one disk revolution) to/from RAM containing an image of the read/written track. Thus for reading just one sector a drive copies a whole track to RAM and reading of all subsequent sectors (if necessary) is performed from drive RAM instead of disk surface. Identical operations are performed during recording. During sector recording a drive reads a track, modifies it in RAM and writes the whole track back to disk.
Configuration tables and settings of hard drives contain information about logical and physical structure of disk space. Those tables enable PCBs, which are identical for the whole drive family, to self-adjust for a certain drive model. As a matter of fact, during design of a certain model like, for example, a 80 Gb drive based on two disks it allows to produce automatically a “half-size” model with 40 Gb capacity based on one disk and “quarter-size” model with 20 Gb capacity based on one side only. Thus a manufacturer can offer a greater number of models with varied capacity for the market without considerable R&D expenses. Besides, junior models can use disks, which for some reasons are unsuitable for full-size models. E.g. “half-size” models can successfully use magnetic disks with defects on one of their surfaces, etc.
Tables of defects. Modern technology of magnetic disks production does not allow their defect-free manufacture. Heterogeneity of media material, polishing defects, admixtures during magnetic layer application, etc. result in appearance of areas, where data recording or reading end in errors.
Earlier drives with ST506/412 interface displayed the table of defective tracks as a label on HDA case and any drive had some reserved space, e.g. HDD ST225 (20 Mb) had actual capacity of 21,5 Mb, i.e. 1,5 Mb extra were allocated for defective sectors and tracks. Modern HDDs also have extra capacity, but it is hidden from users and only drive microcontroller can access it. A portion of that extra space is allocated to HDD firmware, configuration tables, S.M.A.R.T. counters, factory information about a HDD, tables of defects, etc. The remaining part is held in reserve for substitution of defective sectors with the reserved ones.
Tables of defects are filled by the manufacturer during internal factory testing. Numbers of all discovered BAD sectors are added into a table. Such procedure is called updating (relocation) of defects (UPDATE DEFECT). After that if a defective sector is addressed during work with a HDD, the drive itself will redirect the request to a reserved sector. Therefore all modern drives newly arriving from the manufacturing factories have no defective sectors.
Most HDD models have two tables of defects: Primary or P-List and Grown or G-List. Primary table is filled at the factory during internal testing - SELFSCAN (intelligent burn-in). Grown list is not filled at the factory; it is designed for addition of defects which appear during drive operation. To enable that functionality, the list of user commands practically in all HDDs contains the “assign” command replacing a defective sector with a reserved one. The command is used by numerous test utilities including those recommended by the manufacturers for operations over drives with BAD sectors. Western Digital drives have a Data Lifeguard system, which performs automatic substitution of defective sectors while a drive is idle. In order to perform the procedure, a drive self-tests its surfaces and transfers user data to a reserved sector marking at that defective sector as BAD; the mechanism of defect relocation is identical to the ”assign” command. Manufacturers of Fujitsu, Quantum, Maxtor, and IBM drives implemented a mechanism of automatic defect relocation during the recording process. Thus if data is recorded to a defective sector, a drive itself will redirect such request to the reserved zone marking at that the defective sector as BAD and adding its number to G-List. Among specialized utilities used for relocation of BAD sectors we can note FUJFMT.EXE for Fujitsu drive, WDDIAG.EXE for Western Digital drives, ShDiag.exe offered by Samsung, etc.
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| Two mechanisms of defect relocation |
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When the substitution (Assign) mechanism is used in a drive the latter records to the ID field of a BAD sector the flag of the relocated sector and writes to the data field the number of the reserved sector, i.e. the one, which should be accessed for data recording or reading. As a rule, it is the first available sector after user data area. (figure 2.).
Figure 2. Method of rededicated sector.
During data read/write operations accessing the defective sector drive controller will read the flag and assigned address and reposition the heads to the reserved zone in order to perform reading/writing from/to a good sector. Defective sectors in that case will disappear, but the drive will perform positioning to the reserved area each time it has to address a defective sector. The procedure is accompanied with clicking sounds and slight slow-down. The “Assign” procedure allows relocation only for defects in data fields. Errors pertaining to corruption of ID fields or servo fields cannot be relocated using the “Assign” method.
Another mechanism used for hiding defective sectors at manufacturing factories is skipping of defective sectors. When that method is used, the defective sector is skipped, its number is assigned to the following sector (and so on), and the last sector is shifted to the reserved zone. (figure 3.).
Figure 3. Method of missing sector.
Such method of sector hiding disrupts the continuous integrity of low-level format; the system of LBA conversion to PCHS should also take into account BAD sectors while skipping them. Therefore the method requires obligatory recalculation of translator tables and low-level formatting making it impossible to preserve user data if the method is employed. Exactly for that reason the said method of relocation is applied only in special factory mode of drive operation. It is used in the FUJFMT.EXE utility designed for relocation of defects in FUJITSU drives.
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| Logical structure of disk space |
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Considerable part of disk space in modern drives is hidden from users; it contains service data and an area reserved for substitution instead of defective sectors in a HDD. In normal operation mode it is accessible by drive microcontroller only. Users may access the working area frequently called logical disk space and it is exactly the same capacity as the value indicated in the characteristics of a certain model. Access to the working area represented by a continuous chain of logical sectors is performed in LBA notation from 0 to N. Connection between the logical disk space and physical disk format is established through a special program, i.e. a translator, which takes into account physical format, zone allocation as well as defective sectors and tracks to be skipped during operation.
Access to firmware zone is possible only in a special drive operation mode, i.e. factory mode. A drive is switched into that mode by a key command opening access to an additional set of factory commands. Those commands are used for such operations as reading/writing of firmware zone sectors, obtaining a map with locations of modules and tables in firmware zone, access to zone allocation table, conversion of LBA into PCHS and vice versa, launch of low-level format, reading/writing to/from Flash ROM and some other actions.
In the process of HDD design developers define firmware data required for drive operation as well as the number of cylinders occupied by firmware; therefore zero logical cylinder is the first free cylinder following the last cylinder occupied by firmware area. (See figure 4.) The structure of disk space may vary with different HDD models.
Figure 4. Logical structure of disk space.
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Equipment
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