Technical Guide
How does a Hard Disk 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 drives are available for devices that require a micro-drive.
The spindle is rotated by an electric motor, and this cause the platter to spin. The speed at which the platter spins is measured in RPM and a higher speed is usually reflective of a higher performance, disk, in terms of data writing and reading.
The magnetic media holds the binary data as with tapes and floppy disks. 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 drive with 5 platters would have 10 heads.
When the disk is not in use, the heads are ‘parked’, usually at the outside edge of the platter.
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 OS 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. The shorter the seek time and latency, the faster the drive can do its work.
When the drive electronics determine that a head is over the correct sector to write the data, the drive sends electrical pulses to that head. The pulses produce a magnetic field that alters the magnetic surface of the platter. The variations recorded there now represent the data.
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 specks that represent your data in the right sector and track pass under the read head, 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 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 o go through dozens of firmware revisions during the model’s lifecycle.
If a PCB becomes damaged, or a component burns out, it is possible to either mend or replace this PCB. Repairing the PCB is easier than replacing the PCB as a replacement would need to be found with identical firmware. With an HDD that is over 6 months old, the procurement of an identical revision can be time consuming and difficult.
What is a Head Crash?
A ‘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.
A 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.
Since most modern 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 in the media that is scraped off the platter is of course unrecoverable, and because of the way that data is stored, randomly over a disk surface, this data may be whole files or parts of many files.
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.