How Data is Stored on a Hard Disk

The amount of data stored on hard disk drives has got to the point where almost every aspect of business and part of home life is stored on computer. For example;

• Business
  • Accounts systems
  • Personnel data
  • Customer databases
  • Supplier databases
  • Quality management systems
  • Stock management
  • Email systems
  • Security camera systems
  • Correspondence
  • Graphics
  • Company Proprietary Software and Data
• Home Users
  • Photographs
  • College, School or University work
  • Music
  • Video
  • Genealogy
  • Emails

Principles of magnetic data storage

Data is stored on hard disk as magnetic 'flux reversal patterns'. As we learnt at school, a magnet consists of a North (N) and South (S). It is not the intention of this document to teach magnetism, however a good link can be found on Wikipedia .

The component of a hard disk where the actual data is stored is called a 'Platter' that has a thin magnetic coating applied during manufacture. It is within this coating that the data is stored.

Data is stored on the platters as a series of 'Flux Reversals' - For clarity, a magnet is represented as '[N-S]'. So data is represented as either [N-S] or [S-N] and these translate to binary 1 or 0 depending which direction the flux is pointing and how the manufacturer determines 1 and 0 from the reversal. For example a series of data may appear as the following sequence of flux reversals. [N-S][N-S][S-N][N-S] which for our example, would represent binary 1101

Reading data from a hard disk

When reading or writing data on modern hard drives, the heads 'fly' a few nanometers (1 nm= 0.0000001mm) above the surface of the platter. During a read sequence, the head enters the magnetic flux patterns just above the surface of the rotating platter of the bit that is required to be read. This magnetic flux induces a resistance change in the materials in the read element of the head, depending on the magnetic flux direction that is [N-S] or [S-N]. This value of the resistance change is then converted to binary 0 or 1 to represent the data.

Writing data to a hard disk

The write operation is the reverse, a pattern of electrical pulses representing the data pass through a coil in the write element of the head, producing a pattern of magnetic fields at a gap in the head. These magnetic fields alter the magnetic orientation of the bit regions on the disk, so the bits now represent the data.

Perpendicular and Longitudinal Storage Methods

Up until recently, hard disks used the longitudinal method of recoding data. This has recently be superceded with perpendicular recording technology. This has revolutionised hard disk storage capacity and at the time of this document hard disks of 1 Terrabyte (1,000 Giga bytes) are readily available to consumers.

Problems that can happen with platters

Platters are the most critical part of the hard disk, other components can be replaced but it is the platter that stores the data. If the platter becomes damaged, then access to the data is lost.

Typical Physical Problems with platters

• Bad sectors. Where the data in one of more areas of the drive is no longer readable.
• Head Crash . Where the heads come into contact with the platter and physically damage the magnetic coating.
• Thermal damage. Where excessive heat (typically that found in a fire, or caused by excessive heat generated by worn motor bearings) renders areas of the platter de-magnetised resulting in inability to read data

Data Recovery from Platter Damaged Hard Disks

In some cases where the damage is minimal, data recovery is possible. It will involve cleanroom work to first determine the extent of the damage then to change any other damaged components, e.g. Heads or Motor. Imaging of the drive will be a slow process and can often take several weeks. The primary objective is to image around the damaged areas and determine recoverability of critical data. Where the critical data is still undiscovered, careful attempts are made at recovering readable areas from within the damaged zone.

Symptoms of Platter Damage

Without opening the drive it is difficult to determine the extent of damage and hard disks should NOT be opened outside of a controller cleanroom environment. Usually though, there are other indications of platter damage. These can consist of;

• Clicking noises. (see note below about clicking noises)
• Loud grinding type noises as the drive spins.
• Static Electricity (or ESD). ESD, believe it or not, can cause platter damage. What happens is as the ESD discharged through the head, small particles of the head material 'blow out' and can end up on the platter, when the heads hit these parts, they can cause severe head crash.

Also, when a head blows out, it can also misalign the damaged head to the point where it starts grinding into the platter surface, creating concentric rings where the data once was as in the photograph below.

 

Note that clicking or knocking noises can also be symptoms of other less serious problems such as failed head amplifier and does not always mean that there is platter damage. It is more likely to be platter damage if, for example an external hard drive was dropped or a computer moved whilst powered up.

 

Generally damage like this is unrecoverable, because the material that has been ground off settles inside the hard disk - including the platter itself. This in turn causes more head crashes. Also, because the surface is no longer smooth, when the drive starts to rotate during a power up, the worn platter areas work in a similar fashion to that of a grinder rendering the replacement heads useless.

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