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Your Question:

Why is Migrating Disk Platters Such a Big Deal?
        What makes the service so expensive
                                when this action is needed?

Our Answer:

From a conceptual standpoint, removing hard drive disk platters from one drive and placing them into another should be a simple, seemingly trivial procedure.  In practice however, this is far from the actual case.  In reality, the process nearly always involves what can be viewed as "major surgery" on the drive, and data recovery experts do everything they can to avoid it.  Nonetheless, when the spindle motor bearing of a hard drive fails and subsequently seizes in such a way that it will never again permit rotation, there is no alternative.

With present-day hard drives there are multiple reasons for the substantial difficulties in carrying out what is called "platter migration".  One category of concern is purely mechanical, another is dealing with a design technique that is used in today's hard drives to increase the amount of data that can be recorded within a given amount of media space (areal density).

Mechanical Challenges

Hard drive with platters removed.
Platter migration is major surgery when required for data recovery.

Hard disk drives have long relied upon a level of precision that is generally incomprehensible to nearly everyone but scientists and mechanical engineers.  This characteristic combines with another factor — there is nothing within hard drive design to support removal of platters.  Designers expect the mechanical integrity of the head and disk assembly to remain securely and absolutely intact for the life of the device, and the design of the device, enabling its ability to store and retrieve data, is completely dependent on this integrity.  As time has gone on and drives have offered larger and larger data storage capacities, the enabling precision factors have increased in proportion.  Mechanical factors pertaining to position and balance require adherence to mechanical tolerances expressed in nano-inches (billionths of an inch) and in some cases are so demanding that maintaining the required precision cannot be accomplished in any conventionally available way.  Whenever that is the case, the data cannot be recovered.

Adapted Information

Beyond the purely mechanical issues, is a further, entirely separate issue.  Hard drive platter migration and adapted information, stored as data upon the recording medium (not the user's data), combine to give rise to an expensive recovery service experience.  The following description may be a bit heavy on the technical side, and likely makes a fair amount of knowledge about hard disk drive operational technology requisite.

Among the difficulties with spindle motor replacement on higher capacity (especially >400Gb) hard drives has do with an excessive time resource allocation for what boils down to intensive manual labor by the recovery engineer.  That is always costly.  Because the minutely variant values of this drive unique, drive specific information is adapted for each case to the particular combined physical characteristics of the components used to build that drive, when a component such as the motor and bearing assembly is replaced, this value is degraded because information stays unchanged of course, but is then no longer relevant to its new "environment".  Therefore the information can no longer efficiently perform its function because it is applicable to a configuration that no longer exists.  Due to loss of this value, extraction of data requires of the recovery engineer precise, manual manipulation of the magnetic field that must be present to detect data, in order to compensate for the automatic control normally provided by the adapted information.

The "adapted information" is data recorded by the drive about itself, and is one of many capacity enablers of today's ultra high areal density disk drives.  This feature pertains to parameters of write and read operations that require variation of write current and read bias compensation owing to certain dynamical changes required in the magnetic field strength needed to effectively record (write) and detect (read) flux reversals recorded to and from the media.  The way the media interacts with a R/W head depends greatly on its radial positioning over the disk platter surface.

For background it must be understood that the speed of media motion under the head affects signal strength, and similarly, the flying altitude (a.k.a. air-bearing thickness) of a R/W head over the media surface affects signal strength.  Both flying height and media speed are thus closely related, and represent parameters that vary continuously, from the beginning, to the end of the platter's recording space.  Additionally, the media's magnetic coercivity and physical flatness are two more parameters that vary minutely from one physical location to another.  As variation occurs in these physical parameters, the electronic apparatus must be adjusted or adapted to accommodate.  Adapted information is data that performs this function.

The adapted information is recorded on the media to control the level of compensations needed to adjust write current and read bias for the dynamical field force present in any given media surface location; the locations can be defined down to the sector level.  The amplitude of characteristics that come into play vary minutely from place to physical place upon the recording surface.  The variations are random, thus making each disk drive platter surface unique, and it is this "adapted information" that enables compensation for the uniqueness.

The above described scheme is but one of many ingenious ploys of hard disk drive technology and engineering that help lay the groundwork for data storage devices that achieve greater and greater areal density and thus are capable of storing ever increasing volumes of user data.

Contributed by S.E. Fowler

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