Hard Drive Platter Storage Architecture

One of the misconceptions about the low-level data storage is that the 0’s and 1’s are physically stored on the drive platter. The data is however encoded before being written to the platter as a wave form testtest.

Prior to writing the data, it is randomised, which eliminates repeated patterns, which could cause problems whereby the Error Correction Code (ECC) becomes confused. As the drive firmware handles this automatically, in most cases it has no bearing on the data recovery procedure.

Physical Data Layout and Servo Data

Current hard drive technology lays the data down in a set of concentric tracks, around which servo tracking information is also stored, which the read/write head uses to ensure the alignment it correctly maintained. Each sector of store data also has an overhead associated with it, including Sync data, Address Marker, CRC and ECC, all of which are essential for correctly storing and accessing the data.

In the future, this tracking information may however be stored on a separate layer, allowing more data to be stored while improving the efficiency of maintaining the read/write head alignment. Future improvements may come from not only storing the track servo data as a separate layer, but also creating additional data layers on a single platter surface, which are accessed by focusing the read/write head to a different depth as required.

Drive Error Codes and Diagnostics

An extremely important part of the operation of a hard disk drive is how it handles any errors and how it interacts with the host computer system. Although called error codes, they also indicate if the drive is ready, busy or received a data request to or from the host computer.

In most cases, any real errors will be handled by the drive firmware, such as automatically mapping an unreadable sector to the spare area of the drive. If an error code is passed back from the drive to the host computer, it is essential that the operating system handles this in a sensible manner, without causing a system crash, which could result in unsaved worked being lost.

Data Recovery and ECC

At one time, it was possible to read the data sectors without invoking error correction, but even for data recovery purposes this proved to be of little use and is only available for drives less than 137GB in volume. By default, the drive will make several attempts to read each sector using different methods before it will timeout and return an error.

There are several techniques which our data recovery specialists can employ to read a previous unreadable bad sector, although each attempt may risk causing further damage to the drive if the problem caused a physical issue. Therefore, our data recovery engineers will maximise the yield of good data sectors before making attempts to read sectors which failed to be read correctly during the imaging process. In a number of cases our data recovery engineers have been able to recover all the sectors from a failing hard drive, despite a sector being unreadable during the sequential imaging process.

Hard Disk Drive System Area

An extremely important, but rarely mentioned part of a hard disk is the System Area (SA) which contains vital information required for the normal operation of the drive. Any damage to the System Area can cause problems, ranging from small levels of erratic behaviour to the drive being totally inaccessible.

If the System Area on a hard drive becomes damaged, the drive must be sent for professional data recovery, in order for the data to be accessed. Making attempts to recover the data yourself may, depending upon the problem, make the situation worse, which could result in a total loss of data.

System Area Information

The importance of the System Area becomes apparent by examining the data stored in it, which includes the system logs, smart data, drive serial and model numbers, the defect lists, firmware, test routines, recalibration code, translation data, security information and other important information essential for the correct operation of the drive.

The corruption of even a single part of the data within the System Area may cause the drive to malfunction. Depending upon the exact nature of the corruption, it may be possible that the drive will still, function behave in an erratic manner, or it may total fail operate at all.

Important Information About the System Area

In most drives, there is at least two copies of the System Area, which is usually located on different platters, in most cases at the extreme out edge. This method of storing one or more backups of the System Area, should allow the drive to overcome corruption of the primary copy, which allows the disk to continue operating correctly.

There is no unified or standardised format used for storing the System Area information, which can be completely different for between drives in the same family. Often, even a difference in the firmware version for a particular make, model and capacity of drive will lead to the System Area data being different. This can cause problems in sourcing the correct donor drive when the firmware or other System Area information has been damaged, causing the drive not to function correctly.

System Area Damage and Data Recovery

In the event of System Area damage, the drive will require professional data recovery to gain access to the data. The recovery process requires another drive of the same model, capacity and firmware to be sourced as a donor.

The process requires complex electronic work to complete the hardware modifications essential to allow the drive to be made operational again. This work should only be undertaken by a qualified data recovery hardware specialist, in order to ensure the drive is not damaged further.

Hard Disk Drive Startup

The sound a hard disk drive makes when you first turn your computer or external storage device on is well-known, with the motor starting to spin, closely followed by a few clicking sounds. If all is fine with the drive, it will quickly settle down with the platters continuing to spin, waiting for the drive to be accessed.

Any change in these sounds, such a different pitch to the spinning or extra clicks from the read/write head should be taken as warnings that the drive may not be completely healthy. If the drive is heard to spin down or it makes a lot of clicking sounds without the computer being able to boot up, the computer should be turned off and the drive sent for data recovery.

The Disk Drive Boot Sequence

When power is applied to a hard disk drive, it checks the status of each chip contained on the controller board, in order to ensure all the electronics are functioning correctly. The drive then performs a self-check of its other components.

If the controller board chips check and the self-test of the other components passes, the drive spindle motor is started, which spins the platters causing debris to be removed from the surface. The spinning platters causes movement of the air or gas contained with the drive to flow, which creates the air bearing, essential for keeping the read/write heads flying at the required height. This airflow causes the guard protecting the read/write heads, which are parked when the drive is powered off, is moved once the air is moving fast enough to ensure they will not come into contact with the platters. When the drive is powered off, the spinning of the platters is used to ensure the read/write heads are moved back to the parking position.

The read/write heads check the servo timings which allows them to located the exact position of the system area and other sections of the drive. The system area is then read from the drive platters, which can include additional firmware and overlays, which is most often stored at the other edge of the platters.

Problems During the Boot Sequence

If the drive servo timings, the system area or the read/write heads are damaged it will result in repeated clicking noises as an attempt is made to access the data. It is also possible that if a hard disk drive is powered up and powered of several times in a very short period of time, that the read/write heads can fail to move back to their parking position, instead becoming stuck to the surface of the platters, which will most likely damage the read/write heads as well as the magnetic recording layer.

Corruption of the firmware or system area information can result in a failure of the drive to boot up. These may result in the drive clicking repeatedly or the platters may spin down. In either case, the drive will probably not be detected by the computer BIOS.

Disk Boot Failure and Data Recovery

If your hard disk drive fails to start correctly and makes repeated clicking noises, or the platters can be head to spin down, you will require professional data recovery. The drive will require the use of donor parts to overcome the issues before a sector-by-sector image of the drive can be secured.

In either situation is important that the drive is powered off as continually attempting to boot the drive up can result in further damage occurring, which could ultimately result in your data files being lost.

Solid State Hybrid Drive Technology

Solid State Hybrid Drives (SSHD) combine hard disk drive (HDD) technology with NAND flash Solid-State Drive (SSD) technology. This provides a cost effective compromise between the cheap storage capacity of traditional hard disks and the speed of SSDs. The role of the SSD is to act as a cache by storing frequently accessed data, such as the operating system files.

The first generation of hybrid drives were introduced in 2007 by Seagate and Samsung, with only 128MB or 256MB of flash memory. The first use of the term Solid State Hybrid Drive came in 2010 when Seagate introduced a 500GB drive with 4GB of integrated NAND flash memory. As with all hard disk drive technology problems can occur, from physical failures to logical damage of the data stored on the drive. Although they contain an SSD element, data recovery from a hybrid drive is very similar to a traditional hard disk drive.

Dual Drive Hybrid System

Many systems combine an SSD and an HDD which is often referred to as a dual drive hybrid system, although this is a misnomer. It is in reality usually an SSD upon which the operating system is installed, with the larger hard disk drive used for data storage.

In 2013 Western Digital produced a dual drive unit where a single 2.5” unit contains a 120GB solid state drive with a 1TB hard disk drive, as yet the only device of this type. These are separate drives for use within a laptop with only a single drive bay. Software hybrid volumes can also be created, where an SSD device is used to act as a cache for a traditional hard disk, such as bcache and dm-cache under Linux and Apple’s Fusion Drive.

Hybrid Drive Internals

The NAND flash solid state drive element of the hybrid drive is usually only a few gigabytes in capacity, which is usually enough to act as a cache for the most frequently accessed data used by the operating system. Improved performance is achieved by placing so-called “hot data” in the NAND flash memory in the drive. Decisions regarding the placement of this data is done either entirely by the device, known as self-optimised mode, or through hints provided by the operated system, called host-hinted mode. Support for host-hinted mode in Windows 8.1 and patches for the Linux kernel were made available in 2014.

Hybrid Drive Data Recovery

As with any storage device physical failures can occur, either to the electronics or with the components used in the traditional hard disk drive element of the device. Although failure of the controller board and the traditional hard disk drive are more likely, it is also possible for the solid state drive section to fail. It is unclear whether a failure of the solid state drive will render the hybrid drive unusable, or if it will just operate as a standard hard disk drive, with no caching available.

Logical failures through damage to the on-disk partition structures or the file system, either through user error or data corruption are no less likely than with any other type of storage device. If you hybrid drive fails, DiskEng are able to provide the data recovery solution to suit your requirements.

Hard Disk Recording Technology

Over the last few years with magnetic recording densities fast approaching the limits of areal density and the rapid increase in SSD capacities, the days of the hard disk drive have come into question. Prices and further advances in technology are however ensuring that hard disk drives will remain the most common mass storage media for many years to come.

In spite of the continual research and development being undertaken by hard disk drive manufacturers, the techniques used in the data recovery process are relatively unchanged. This is in contrast to SSD technology where the rapid development of proprietary technology makes data recovery a moving target, requiring new techniques to be developed.

Perpendicular Magnetic Recording

The first major advance allowing a significant increase in drive capacities was the development of the perpendicular magnetic recording method (PMR). The alignment of the data using this technique is alignment perpendicular to the surface of the platter, rather than the traditional horizontal method, which allowed the recording density to be increased by several orders of magnitude.

The addition of a heating element to the read/write head, along with separate a read and write head, allows the magnetic field to be focussed, which also allowed a further increase in density. This technology is however reaching a physical limit.

Shingled Recording Method

One method to overcome the problem of the limit in density has been the development of the shingled recording method, whereby the data is overlapped. This however comes at a cost to write speed, as all data in a 4kB sector must be rewritten when any changes are necessary. These drives are therefore marketed for archive purposes and not recommended for general everyday use.

Layered Recorded Method

A paper has recently been released detailing a proposed method of recording separate layers with the magnetic medium, which would allow a significant increase in recording density. The ability to access data on the separate layers is realised by using different microwave magnetic frequencies. This represents the next biggest step in hard disk capacity which can be manufactured, while providing the best performance available from the technology.

Other Techniques to Increase Capacity

Flutter from spinning platters imposes another limitation, this time on the number of platters which can be used and their thickness. One method which allows for thinner platters without increasing the flutter to the point where the read/write heads impact with the surface is to fill the drive with helium. The use of an inert gas allows thinner platters to be used while allowing the maximum number possible in the form factor.

A proposal to also go back to using the 5.25” form factor is under review, which may also allow a further increase in capacity. The future for the hard disk drive appears to have many years, especially as SSD’s are so much expensive per GB. Data recovery from traditional hard disk drives is easier than SSD’s with a much higher rate of success.

Hard Disk Controller

The controller board on a hard disk drive is a multifunctional component required to do a variety of tasks which must be performed with perfectly for the otherwise erratic behaviour could occur. The main function of the controller board is to act as the interface between the hard disk drive and the host adaptor of the computer it is attached to. It must also maintain the precise platter rotation speed, control the read/write heads and maintain the operational parameters, such as the defect lists.

Modern disk drives are manufactured with an integrated disk controller board, but this was not always the case, with separate add-on cards inserted into a peripheral slot. The add-in cards were paired with the hard disk drive, which could lead to problems if the wrong card was used. The controller card is extremely important for data recovery, as it must function correctly in order to recover the data correctly.

Controller Failures

The main causes of failure are due to overheating or a power surge, which can damage the components on the controller board. Sometimes this damage may not be sufficient to stop the drive from working, but its behaviour may become erratic which could potentially lead to further issues.

Damage to the firmware or maintenance area can cause the drive to go offline randomly and power down. Controller card failures of all kinds should be addressed by a professional data recovery company, as specialist equipment is required to rebuild the drive into the correct working condition.

Function of Defect List

Each hard disk drive when manufactured usually has a number of defects, the location of which must is identified during the low-level factory format process and stored in the primary defect list (P-List). During a hard disk drive’s operational lifetime it will develop additional defects, which when detected are mark in the grown defect list (G-List) and swapped to a spare sector.

These defect lists are extremely important as they ensure the correct sectors are access for each read or write operation. A failure of the defect list will result in a wrong sectors being returned, which leads to a failure of the file system, and causes complications in the data recovery process.

Data Recovery and Controller Boards

Information about swapping controller boards following the failure of a hard drive is readily available on the internet. The drive firmware revision of a replacement board must match that of the donor board. However, just replacing the controller board will not make the drive work correctly, as the chip containing all the drive specific information must also be transferred. It is this process which requires expertise and specialist equipment, which mean that a professional data recovery company, such as DiskEng, are best qualified to undertake this work.

Following the failure of a hard disk drive controller board, it is best practise to power the computer down and consult with a data recovery company. Repeated attempts to recover the data through powering it up could result in additional failures occurring.

Hard Disk Read/Write Heads

The role of the read/write head is that of a transducer, converting electrical signals to magnetic signals when writing and from magnetic signals back to electrical signals when reading. The read/write heads are tiny electromagnets which converts the zeros and ones, signifying each bit, to and from the patterns of magnetic flux reversals recorded on the hard disk drive platters.

The read/write heads are often overlooked, but are probably the most sophisticated component in a hard disk drive. A failure of the read/write heads, or a head crash will require professional data recovery services in order to overcome the problem. Data recovery in these situations can be a complex process, particularly if the heads have crashed into the surface of the platters, as it will cause a lot of collateral damage to the magnetic recording layer.

Read/Write Head Types

The older conventional read/write heads, such as ferrite, metal-in-gap (MIG) and thin film, make use of the two main principles of electromagnetics. Firstly by applying electrical current through a coil a magnetic field is produced, used when writing the disk platter. Secondly, is that a magnetic field in close proximity to a coil will induced an electrical current to flow, which is used when reading from the disk platter.

Magnetoresistance (MR) and giant magnetoresistance (GMR) heads do not make use of the induced current in the coil for reading data. They rely on the principle of magnetoresistance, whereby the resistance of the material changes when subject to different magnetic fields. This allowed a much higher recording density leading to a sharp rise in the maximum capacity of hard disk drives.  Two separate heads could be used each optimised for their purpose, unlike the conventional read/write heads where a compromise was required; this allowed a further increase in recording density.

Tunnelling magnetoresistance (TMR) whereby microscopic heating coils where used to control the shape of the transducer region of the head, which allowed a further increase in hard disk capacity. This was closely followed by a move to Perpendicular magnetic recording (PMR), in which the magnetic field is recorded perpendicular to the surface of the platter, resulting in a huge increase magnetic density, leading to a sharp rise to disk capacities in the terabyte range.

Read/Write Head Air Bearings

The read/write heads must fly at a constant “flying height” above the surface of the disk platters, to ensure that they will not impact with them, resulting in a head crash. This is achieved by through the use of an air bearing, where air is drawn through the head by the action of the spinning platters, causing an area of pressure allowing the read/write head to float on a cushion of air.

In spite of the “flying height” being decreased significantly as recording density has increased, the read/write heads are now less likely to crash into the disk surface. Most head crash events are now caused as a result of an impact, such as a dropped external USB drive or laptop.

Data Recovery and Read/Write Heads

Any failure of the read/write heads will require the drive to be rebuilt by a professional data recovery company, such as DiskEng. If a head crash has occurred the platters will have also suffered damage, and part of the recovery process requires the imaging process to avoid these areas where possible, as they will probably destroy the replacement read/write heads, and also cause further damage to occur.

Hard Disk Spindle Motor

The importance of the spindle motor contained within a hard disk is often overlooked, with most people only think about the speed at which is rotates the drive platters. Not only is do these need to spin at a constant speed to a high degree of precision, but they must also be quiet, and not lead to the platter being about to move out of alignment, which could result in the destruction of the magnetic recording surface, the read/write heads, or even the platters themselves.

The spindle motor is required to operate continuously at a constant rotational speed, while avoiding any vibration which could result in a resonance, whereby the platters could start to move out of alignment. It is the workhorse, and a failure of the spindle motor will always require professional data recovery, to rebuild the drive, in order to recover the data.

Traditional Ball Bearing Motors

Up until the early 2000’s, almost every hard disk drive spindle motor contained ball bearings. These would support the spindle motor and ensure the platters were centrally located. Ball bearings however have three design limitations for use within hard drives: rotational speed, storage capacity and noise issues.

Ball bearings can only rotate up to a certain speed, before they will begin to overheat, causing problems with the lubricants which will outgas. They also have a limited lifespan, which will result in a limited lifespan.

The storage capacity in the drive was limited by the magnetic density of the recording layer, but increase density saw a requirement for lower read/write head flying distances, as well as higher precision required in platter alignment. However well manufactured ball bearings are, they will usually contain an inherent wobble, which limited the precision of alignment.

With hard drives being widespread in the office and home environment, it was also important to find a solution which would produce less noise.

Fluid Bearing Motors

A fluid bearing, also known as a liquid bearing, is essentially a small quantity of lubricant which is trapped with a high precision machined housing. The use of fluid bearings has addressed the three issues that were apparent with ball bearing motor spindles, leading to higher speeds and magnetic recording density, along with a reduction in noise.

With no mechanical connection between the platters and the motor, vibrations and noises are no longer transmitted between the two, leading to less chance of a resonant vibration occurring. The fluid in the bearing also acts are a damper, helping to reduce the levels of noise.

Fluid Bearings and Data Recovery

Although an impact event is still highly likely to cause a failure of the spindle motor, the extent of the problems have been lessened, as fluid bearings are more shock resistant than ball bearing motor spindles.

It is essential though, that if a spindle motor is damaged, that the drive is not powered up, as it may lead to further failures, such as the read/write heads coming into contact with the hard disk platters. Data recovery from a failed spindle motor requires the drive to be rebuilt before it can be powered up, in order to avoid causing widespread damage to the magnetic recording surfaces.

Disk Platter Fabrication

Data is stored on the rotating platters used in hard drives, which are coated with carefully deposited layers of magnetic storage media. It is due to the rigid nature of these platters that the name hard disk drive originated.

It is common for multiple platters to be mounted onto a single spindle, upon which the data can be stored using both surfaces. The data upon a platter is accessed using a pair of read/write heads, which fly at a specified distance, just above the surface of the recording medium. If these platters are damaged in any way, this will severely increase the complexity of the data recovery, so it is important that they are free of any contaminants.

Common Substrate Material

It is essential that the material used to manufacture hard disk platters is stable and does not interact with the recording medium at the magnetic level. For a long time it was traditional to manufacture these platters from aluminium. However as technology has progressed, allowing higher recording density, the necessity for the read/write heads to fly at lower distances, has required smoother substrates to be used. Glass was initially used, but now many platters are made from a glass and ceramic composite.

These glass and ceramic materials also give an enhanced rigidity while using thinner platters, which allows for more platters to be held in a hard drive. Glass and ceramic substrates also expand less than aluminium at higher temperatures, allowing for higher engineering tolerances.

Layer Deposition & Polishing

Although the platter substrates are highly polished, they are still not smooth enough, so it is essential to deposit a hard Nickel Phosphate layer, which after polishing provides a perfectly smooth surface. The roughness as a result of this process is less than a tenth of a nanometre; which is approximately the size of an atom.

Many different complex layers are deposited upon this base layer, which increases the roughness to about four tenths of a nanometre. This is the maximum permissible roughness allowed for the reliable operation of read/write heads which fly above the platter at a distance of two nanometres.

Data Storage Layers & Protection

The traditional magnetic data storage layers were made from a composite of iron and nickel. Technology has since move to a storage layer which is a cobalt, nickel and iron alloy. This introduction of cobalt allows for a much greater control of the magnetic orientation, while also providing a much better signal to noise ratio.

The magnetic data storage layer is soft, so it is essential that a two nanometre layer of carbon overcoat is deposited. This is applied using either ion-beam or plasma-enhanced chemical vapour deposition techniques. A one nanometre layer of lubricant is then coated over the top. To remove any asperities (microscopic unevenness) it is important for the platters to undergo tape burnish and head burnish processes, which also removes any loose particles.

The final process is to test the platters by placing them in a glide test rig, which certifies that there are no asperities present on the platter surface, which ensures that the read/write heads will not crash into the disk surface.

Hard Drive Bad Sector Mapping

It is a fact of hard disk drive manufacture, that no matter how well prepared the platters are, defects causing bad sectors will always be an issue at some point during their lifespan.  It is important to locate any bad sectors when the hard disk drive is factory formatted, and keep track of any which occur during use.

The location of all bad sectors is important for data recovery purposes, especially those detected during the factory format process. Each hard disk drive also contains a set of spare sectors, which are used to remap bad sectors detected during normal operation. The hard drive firmware usually handles this process automatically, but rare instances a failure can damage this information, which had serious consequences when performing a data recovery.

Factory Format Primary Defect List (P-List)

During the factory format process, any bad sectors detected, are stored in an area called the P-List. This list maps a shift pattern for each bad sector, which is used to ensure that the correct sectors are always accessed. A failure of the P-List would cause the incorrect sectors to be accessed, leading to data corruption, as additional sectors may be inserted into the data image. Fortunately this is very rare, and usually only occurs through the result of bad firmware and excessive heat damage.

If during the factory format process, physical sector 10 was bad, it would be placed into the P-List, and logical sector 10 would point to physical sector 11 on the platter. Each bad sector shifts the difference between the logical and physical sector by one for each defect found.

Dynamic Bad Sector Grown Defect List (G-List)

During normal operation, any bad sectors which are detected will be mapped out to a spare sector, the location of which is stored in the G-List. This is a normal part of the operating procedure for a hard disk drive, and no data is normally lost when this occurs.

There are however, only a finite number of spare sectors available, so eventually these will be used up. Once this happens, any bad sectors subsequently detected, will not be swapped out, and their presence must be reported to the host operating system.

Data Recovery Bad Sector Implications

It is rare for the either of these lists to become corrupted or lost, but the consequences could be serious, particularly if the P-List is damaged. For the purposes of data recovery, this would cause the raw data image to become shifted for each incorrectly indicated bad sector in the P-List. The implications of such a failure are serious for the purposes of data recovery, as the underlying file system structures would not occur in the correct positions.

A failure of the G-List would only result in the loss of each sector, or in the worst cases, old data being read from the original location. The hard disk drive manufacturers therefore have a duty of care, to ensure that this does not occur, as a large scale problem with any type of hard disk drive would have a serious impact on their reputation and the future of the company.