How Do Hard Drives Work?

On a hard disk, data is stored in the magnetic coating of the disk’s platters. The platter is a flat disk of either alloy or glass, with a spindle at the centre. Modern platters generally have a diameter of 3.5” in desktops or 2.5” in laptops, although smaller 1.8” drives are available for devices that require a micro-drive.

Hard Disk ComponentsThe spindle is rotated by an electric motor, and this causes the platter to spin. The speed at which the platter spins is measured in RPM (Rotation Per Minute) and a higher speed is usually reflective of a higher performance disk in terms of data writing and reading. Usual RPM is 5200 and 7200. Hard drives with 10,000, 15,000 RPM normally use SCSI (Small Computer System Interface). SCSI interfaces provide for faster data transmission rates (up to 80 megabytes per second) compared to standard Serial ATA (SATA) and hard drives and you can attach as many devices as you like to a single SCSI port. (SEE FIGURE 1)

The magnetic media holds the binary data. The data is read from the surface of the platter by a set of heads which are fixed so that they can only move between the centre of the platter and the outside edge. The heads are held just above the magnetic media by actuator arms that facilitate this movement across the disk’s platter surface. The heads are not designed to touch the platter surface as physical contact can cause damage to the magnetic media. Each platter has a top side and an underside, and there is usually a head for both. Therefore, a hard disk with 5 platters would have 10 heads. (See figure 2)

Data in the magnetic media is organized into cylinders – concentric tracks on the media that are further divided into sectors. A sector is the smallest allocatable logical unit on a drive and usually, but not always, is 512 bytes in size. When an Operating System such as Windows sends data to the hard drive to be recorded, the drive first processes the data using a complex mathematical formula that adds extra bits to the data. When the data is retrieved, the extra bits allow the drive to detect and correct random errors caused by variations in the drive’s magnetic fields.

Next, the drive moves the heads over the appropriate track on a platter. The time it takes to move the heads is called the seek time. Once over the correct track, the drive waits while the platters rotate the desired sector under the head. The amount of time that takes is called the drive’s latency which is measured in milliseconds. The shorter the seek time and latency, the faster the drive can do its work. Average seek time in the majority of hard disk drives is 10ms.

When the drive electronics determine that a head is over the correct sector to write the data, it sends electrical pulses to that head. The pulses produce a magnetic field that alters the magnetic surface of the platter. The magnetic variations recorded on the surface of the platter, is called “data”. Compare these magnetic variations with the grooves on a vinyl record that is read by the record player needle arm which reproduces the music recorded on it by scratching the grooves on the surface of a spinning record.

Reading data complements the recording process. The drive positions the read portion of the head over the correct track, and then waits for the correct sector to orbit around. When the particular magnetic variations that represent your data in the right, the drive’s electronics detect the small magnetic changes and convert them back into bits. Once the drive checks the bits for errors and fixes any it sees, it sends the data back to the operating system.

What is a board swap?

Inside any Hard Disk Drive, (HDD), there is a Printed Circuit Board, (PCB) that contains the electronics that manages the HDDs activities. Like any other PCB, it contains chips and other components that the manufacturer has designed to allow the HDD to function effectively. Each HDD manufacturer has its own proprietary firmware. Firmware is the chips that contain program instructions and is highly specific to each manufacturer and HDD. Firmware is continually updated and as a result, a given HDD may go through many firmware revisions as the manufacturer attempts to get better and better performance from the HD models that it sells.

It is not unusual for an HDD to go through dozens of firmware revisions during the model’s lifecycle. This why you cannot simply buy an identical hard drive and swap the PCB and expect it to work. This must be done by a data recovery technician who will ensure firmware compatibility and correct power cycle. The wrong firmware can cause the read/write heads to fail and cause physical damage which must be avoided at all costs.

What is a Head Crash?

Head crash occurs when the heads of a hard disk drive touch the rotating platter surface. The head normally rides on a thin film of moving air entrapped at the surface of the platter. Shock to a working hard disk, or even a tiny particle of dirt or other debris can cause the head to bounce against the disk, destroying the thin magnetic coating on the disk. AND THIS MEANS LOSS OF DATA!

Since most new drives spin at rates between 7,200 and 15,000 rpm, the damage caused to the magnetic coating can be extensive. At 7,200 rpm the edge of the platter is travelling at over 74 miles per hour (120 km/h), and as the crashed head drags over the platter surface they generally overheat due to friction, making the drive or at least parts of it unusable until the heads cool. Following a head crash, particles of material scraped free of the drive surface greatly increase the chances of further head crashes or damage to the platters. Data stored on the media that is scraped off will irrecoverable.

The most severe head crashes are the ones where the entire stack of heads, crash on each of the platters in the stack. A violent movement, or shock, to a working hard disk drive usually causes this. The chance of a good recovery in these circumstances is often remote and is generally limited to partial files.

Levels of Complexity in Data Recovery

Logical corruption:
This means that the computer is unable to make sense of the data that is randomly stored across the disk. The HDD loses its logical format and does not show up in the system. Disk utilities can see the drive but it shows it as unallocated space. This is usually caused by the computer’s index system being damaged or corrupted. The data is still there, but the computer is unable to recognise it for what it is, and thereby unable to reconstitute it into a readable document or file. Where structural corruption is the cause of the data loss, your chances of getting all the data back are extremely good. With the use of advanced tools and professional software and disk editing methods, Data Recovery Doctors can return the hard drive to a state that is understood by the computer. The files are most often undamaged after recovery.

Electronics failure:
This means that the external electrical circuitry of the hard drive has failed. Recovery from a hard disk in these cases is possible, as long as a replacement circuit board can be located or the circuitry can be repaired by Data Recovery technicians. This is not as simple as it sounds, as each hard disk may go through many revisions during its life-cycle, and a revision specific for printed circuit board or PCB must located in our parts inventory, or must be ordered from our suppliers.

Mechanical failure:
This means that the internal mechanics of the hard disk drive have failed through internal factors such as age, or minor manufacturing defects, or as a result of external factors such as shock, heat or water. This is more serious than an electronics failure as the internal mechanics within a modern hard disk are very delicate, and have extremely small tolerances. Again specific revision parts are required, and the internal mechanics will need to be mended or replaced in order for the hard drive to be able to read the data again. The hard disk needs to be disassembled in a class 100 clean environment to prevent damage to the disk platters upon which the magnetic media stores the data.

Media damage:
This means that the magnetic media on the surface of the hard disk platters, has become damaged or corrupted. This is mainly caused by what is known as a ‘head-crash’, where the electronic heads that read the data, from the disk surface, actually crash into the spinning platter surface and begin to scrape the media away. Once magnetic media that contains your data is scraped away, and turned into dust, data recovery becomes extremely difficult and expensive. As a computer stores data randomly across a set of platters in a hard disk drive, a relatively minor head crash can damage many files. Whole files and sometimes parts of files can be recovered but it is likely that the quality of the recovery is going to be lower than another type of hardware failure. On many occasions, the media damage is so severe that little valuable data can be retrieved.

Undelete Files:
When a user deletes a file, whether accidentally or intentionally, the actual data is not destroyed, but the computer system now regards that data as no longer required.

The data stored on a hard disk drive, are pieces of the document in random areas of the magnetic media on the hard disk surface. It does this to speed up the time taken to ‘write’ the data. Where ever the ‘heads’ happen to be when the save command is received, they ‘write’ data to the magnetic media. As bits and pieces of the file are stored in different areas, the computer system requires an index or map to be able to put the bits back together again, in the right order, to reconstitute the file. The index is stored in a FAT or ‘File Allocation Table’.

When one deletes a file, the entry in the table is removed, telling the computer that those areas that previously contained a part of a file, are no longer required, and are available for new data to be stored. The computer does NOT go and ‘over-writeÂ’ the original data, so it remains in place until another set of data is randomly stored there. As long as the ‘deleted data’ has not been overwritten by new data, it can be found, reconstituted and recovered. Once deleted, data is over-written by new data, it is virtually impossible to recover it.

If you re-install Windows and realise that your data is missing, stop installing any applications as the more data your write on the disk, the less the chances of data recovery will become.

For more information on data recovery or any data loss situation, please call 0207 516 1077 and speak to Data Recovery Doctor.