A hard drive is made out multiple flat disks called platters which magnetically store the data.
The platters are connected to, and rotate around an axis, called the spindle. The
spindle is used to rotate the platters so that an electromagnetic sensor can read to or write to the platter
while it spins underneith it. This electromagnetic sensor is called a head, and both surfaces of
each platter has their own head. This head can move 1 dimensionally toward or away from the spindle
on what is called an actuator arm. Each head sensor is connected to the hard drive's logic
circuitry which controls disk rotation, buffering, and interfacing with the rest of the computer. This
whole assembly is enclosed in an air tight case to prevent particles from scratching the disk surface.
Platters
Platters are made out what is called a substrate, and this is usually an aluminum alloy. The
substrate has to be durable, lightweight, inflexible, and heat resistant. It has be be able to withstand
constant rotation without warping do to heat, or vibrating. To store the magnetic information, platters
are coated with a magnetic material, which is less than a millimeter thick. This magnetic material will
retain an electrical charge much like what is used in a cassette tape or floppy drive. When the material
is in contact with a magnetic field, it causes the material's atoms to align like a magnet. This stores the
magnetic field, to be read or written overtop of. The magnetic material is bonded to the platters either
through electroplating or a technique called sputtering. On top of the magnetic
material, a protective layering is added to both lubricate the platter and protect the disk from scratches.
Most modern hard drives contain between 1 and 5 platters. Platters are usually 3.5" in diameter to
maximize the efficiency of the 3.5" drive bay. Some hard drives use 5.25" drive bays so their
platters would be up to 5.25" in diameter. In laptops, where space is a major concern, hard drive
platters can be as small as 1" in diameter. The advantages of large platters are their extra surface area
which can increase capacity. The down sides to large platters are that they are more susceptible to
damage due to vibration. More down sides are that they require more power to turn the spindle,
resulting in needed and larger motor which in turn creates more head. The largest concern though is that it is
harder to manufacture large platters that are uniformly flat.
Connectors
On the back of the hard drive there are multiple connecters which are used to by the hard drive. These
usually include power, data interface, jumpers, and LED connection.
The power interface is used to supply power to the hard drive. The power cord is directly
from the power supply to the hard drive, and supplies the hard drive with both +5 volts and
+12 volts.
The data interface is used by the hard drive to exchange data with the system. This
connection runs either to the motherboard or to a controller card. There are multiple interface
formats that can be used.
Jumpers are located on the back of the hard drive to set specific settings for the hard
drive which are needed during detection.
Some older hard drives have an LED connection that will plug into the hard drive
activation LED at the front of the computer case. Most newer hard drives do not
have this feature because it is built into the interface controller.
Read / Write Heads
Read / Write are electromagnetic sensors which run very closely over the surface of the hard drive.
These heads detect the magnetic field around the specific locations on the platters, and sense whether a
certain area has a magnetic charge or not. They convert this pattern into a series of pulses which are
sent to the logic circuitry to be converted to binary and processed by the rest of the system. To store
information, the hard drive logic circuitry sends the heads a series of electronic pulses, and instead of
detecting magnetic fields, the heads create an electromagnetic which realigns the atoms in the magnetic
material.
Artuator Arms
Actuator arms have two main parts. The arm, which is attached to the read/write heads, which is
suspended over the platters, and the actuator. The actuator positions the arm and heads in the correct
location over the platters so that information can be accessed. Changing locations on the hard drive is
very important to hard drive performance, so the fast the actuator is, the lower the latency.
Both sides of each platter have a head and actuator arm and all of the actuator arms for each of the
platters in a hard drive are connected.
Actuators are made out of what is called a voice coil. A voice coil works on the principles of
electromagnetic repulsion and attraction. When the coil is powered, it causes the actuator arm to move
either in or out depending on the strength of the current. The voice coil is a part of a closed loop
feedback system, it will monitor its own performance and accuracy, and make adjustments when
necessary. This closed-loop system used for dynamic placement is called the servo control.
Servo Control
The servo control is part of the actuator. It can dynamically position the read/write heads to
compensate for thermal expansion of the platters. When the read/write heads are moved, they will
periodically check their position to see if they are in the right spot. They do this by reading special data
areas called servo codes. Servo codes can be scattered among the data in non-writeable and
engineered positions, or written completely on one side of each platter. When servo codes are
scattered amongst the data, it is called embedded servo. This practice is most common
because it doesn't effect storage capacity as much. When one side is all servo codes, it is called
dedicated servo. This is used in some high end hard drives because the servo codes are
always accessible, resulting in more precise and reliable for high density drives. The downside to dedicated servo
is that half of the capacity of the hard drive is lost. Servo codes are embedded into the hard drive
during manufacturing and can not be changed.
During thermal recalibration, when the servo information is being read, the hard drive is
inaccessible to other read/write functions. This is not a problem for most situations because hard drive
access is delayed for an insignificant amount. In some situations, such as real-time audio or video recording this pause in hard drive accessibility will cause an error. Some hard
drives have found ways to work around this problem, whether through using large buffers, not using servo control, or by the use of a more efficient servo method. These hard drives are usually more expensive and are marketed under the "audio/visual" or "A/V" name.
Control Circuitry
All hard drives contain simple logic that controls normal hard drive operation. This saves the rest of computer from having to control all of the simple tasks of the hard drive. Of these tasks, one is to process the data which is read off of the platters. Data
is stored on hard drive platters in magnetic fields, and it is the control logic's job to convert the magnetic information read
from the platters into binary information, and vise versa. Another basic task that the hard drive control
logic is responsible for is finding the correct data on the hard disk. These means converting the memory
address which is sent to it by the computer into the correct location on the disk, and moving the heads
to that area for access. Also involved in finding the correct location on the disk, the control circuitry
has to make head skew and cylinder skew adjustments. The higher level functions that
the control circuitry are responsible for are the processing of servo control, data caching, and interfacing with the rest of the computer. Control circuitry is controlled by its firmware, which is stored in a ROM on the hard drive. This contains the software which is necessary for the hard drive to perform basic functions.
Cache
This is a memory buffer which is used to store the data read from or written to the hard drive so that it can be transfered to and from the computer in large packets rather then at the speed it is accessed by the hard drive. Most hard drives contain 512k to 2MB of cache.
