A hard drive, to most of us, just looks like a rectangular device which stores our data. We don’t know how it works, just that all of our precious information is in there somewhere. To get a better understanding of what goes on inside, how they work, and how to recover the data when they stop working, let’s look at a hard drive from the inside.
In order to make the article more relevant we will use a new 1TB Western Digital 3.5″ hard drive and open it up.
The main enclosure, usually black or silver, is what the everyday computer user will know to be the hard drive. It is actually called the hard disk assembly, or HDA. The top side of the HDA is covered by the top cover with a sticker detailing information about the hard drive such as manufacturer, capacity, model, serial number etc.
The underside of the hard drive houses the printed circuit board, or PCB. This is the green electronic board which covers about half of the underside of the drive. The components of the PCB are usually on the inner side of the PCB, protecting them from damage. On the underside the cover for the motor spindle can also be seen.
SATA drives have 2 connectors. The small connector carries data to and from the drive, whilst the larger is the power connector for 5v and 12v lines.
On the PCB are 3 main components. They are the main controller unit, motor controller and onboard memory or cache. The MCU is equivalent to the processor (CPU) of your computer, essentially the brains and computing power of the drive. The motor controller performs the function of spinning up the platter/s inside the drive, and controlling the movement of the voice coil which directs head movement. The memory chip is like the RAM in your computer. It is labelled as ‘cache’ in the hard drive world and stores data being written to, or read from, the drive. The contacts for the heads and motor can also be seen. The ROM information is also found on the PCB and is usually found on an 8-pin IC (chip), other times this information is stored in the MCU itself. The ROM contains a portion of the firmware required to start the drive up, with the rest of the firmware being read from the drive platter itself.
From a data recovery perspective, sometimes the PCB will fail from a power surge or physical damage. To recover the data the PCB will have to be repaired or replaced, but this is not a straightforward process.
The platter, or platters, store your data. They are usually made from aluminium or glass and are covered in several layers of other materials. The platters will spin anywhere between 5,400 RPM to 7,200 RPM in average consumer hard drives. The head stack assembly, or HSA, is the assembly which holds the read and write head/s. These heads fly nanometers over the surface of the drive on sliders, reading and writing your data. The magnets allow the voice coil to function, allowing the HSA to move and is complemented by a bottom magnet below it, in the same way a speaker works. An internal air filter collects any contamination that might reside in the drive such as oil or metallic fragments, all on a micro scale. A plastic ramp is included to hold the heads in place whilst the drive is not in use. Some drives park the heads in the middle of the platters on a special layer, this being seen mainly on older drives.
If the platters are physically damaged it can be a big problem for data recovery, as the damage to the platters can kill any replacement heads used in the data recovery process.
The air filter purifies the air circulating in the drive whilst in operation. It will collect any microscopic fragments of metal or oil which are used in the manufacturing process, or created through wear and tear. A hard drive which has suffered a head crash will have a filter tainted with dark material and appear black or grey. This dark material being the particles of platter/s and head slider/s.
With the top magnet removed, the voicecoil and bottom magnet are visible. The concept of this is the same as the driver in a speaker system, allowing quick and precise movements in either direction. The magnets are neodymium, the strongest form of permanent magnets being able to hold 1,000 times their weight. A stopper, or limiter, is placed at each end of the voicecoil to limit its range of movement. The bearing on which the HSA moves is also shown.
The HSA holds the actual read and write heads at the end of the assembly, fixed to sliders. This particular drive has 6 heads, 3 pairs of 2. For each side of the platter there is one read and write head.
The sliders allow the head assembly to fly over the platters, only a few nanometers above the surface. This is many times thinner than the thickness of a human hair. When the drive is spinning at full speed, the heads will be unloaded from the parking ramp and fly over the platters. The air passing under the sliders at this speed is enough to lift them above the surface of the platters.
The read and write heads work with very weak signals. For these signals, the “ones and zeros”, to be of use the signals need to be amplified. The preamp on the HSA performs this feature, much like the amplifier in a hifi. It amplifies the signals from the head elements to a useable level. When a hard drive suffers a head failure, it is often the case that the preamp has failed whilst the actual heads are still functional. A head failure usually causes a ‘clicking’ sound as the heads try to position them selves, but fail and hit the limiters.