What are the circles on a hard disk platter?

Hard disk drives are data storage devices that use magnetic recording to store and retrieve digital data. The data is stored on circular platters inside the hard drive which spin at high speeds. These platters are made up of a rigid material coated with a magnetic film. The physical circles on the platters are called tracks and are divided into sectors. A read/write head floats slightly above the spinning platter to access the data. In this article, we will take a closer look at the physical circles on the platters that allow hard drives to store data.

Platter Construction

Platters are made of non-magnetic materials like aluminum or glass so that they do not interfere with the magnetic storage of data. These substrates are coated with an extremely thin layer of magnetic material, usually a cobalt alloy, that enables the magnetic recording of data (https://en.wikipedia.org/wiki/Hard_disk_drive_platter). The non-magnetic platter material, such as aluminum or glass, provides a smooth and rigid surface for the magnetic coating to be deposited upon. This allows for greater data densities to be achieved as the read/write heads can fly closer to the platter surface without risk of crashing into irregularities (https://www.pctechguide.com/hard-disks/hard-disk-hard-drive-construction). The substrate and magnetic coating together create a platter that can reliably store data magnetically while rotating at high speeds.


The circles on a hard disk platter are called tracks. Tracks act as circular lanes around the platter where data can be magnetically stored and read from (Track (disk drive) – Wikipedia). A hard disk platter contains many concentric tracks stacked together, with thousands of tracks available on a typical hard disk (Hard Disk Drive Basics | File Recovery).

Each track forms a full circle around the platter and serves as a distinct region for storing data. The presence of multiple tracks provides more physical space for data storage across the surface of the platter. When data is written or read on a hard disk, the read/write head will move between tracks to access the desired location.

Overall, tracks allow a hard disk platter to store more data by dividing the surface into separate circular lanes. The tracks appear as concentric circles on the platter’s surface. Without tracks, the platter would have limited storage capacity and would fill up quickly.


Disk sectors are the smallest physical storage units on a hard disk drive and are typically 512 bytes in size [1]. Tracks are further divided into sectors which are pie slice shaped sections on the disk platter surface [2]. Each sector stores the same amount of data and is addressed using the cylinder number, head number, and sector number [3]. The sectors rotate past the read/write heads which can access the data stored on each sector’s surface.

Reading/Writing Heads

Hard disk drives store and retrieve data using tiny electromagnetic read/write heads that move rapidly over the surface of the spinning platters. The read/write heads are attached to an actuator arm assembly and suspended just above the disk surface by an air bearing generated by the platters’ fast rotation.

There is one read/write head per platter surface. Each head is mounted on a slider that allows the head to float just above the platter surface, with clearance measured in nanometers. The slider is aerodynamically shaped to allow the head to “fly” over the platter surface on a cushion of air.

As the platter rotates at high speed, the read/write head is positioned over a specific track by the actuator arm. The tracks are made up of many smaller sectors where individual bits of data are stored. To access a sector, the actuator arm rapidly moves the head across the radius of the disk to the correct track. Then the head waits for the desired sector to rotate under it before reading or writing data. This combination of radial and rotational head positioning allows data to be accessed rapidly from anywhere on the disk.

The read/write heads contain a tiny coil of wire which generates a magnetic field to magnetize sections on the platter for writing data. For reading, the heads detect small changes in magnetic orientation on the platter as it spins by to decode the binary 1s and 0s. The heads move extremely close to the platter surface at rapid speeds, floating on a thin cushion of air just nanometers above. Modern hard drives use sophisticated actuator mechanisms to position the heads with tremendous speed and accuracy.

Data Density

Data density, or areal density, refers to how much data can be stored on the platter surfaces of a hard disk drive. Modern hard drives have an extremely high density, with tens of thousands of tracks packed very closely together (Track Density and Areal Density). Areal density is calculated by multiplying the number of bits per inch (BPI) by the number of tracks per inch (TPI) (Areal Density: HDD Capacity Explained).

To maximize storage capacity, hard drive manufacturers use a technique called constant angular velocity. This means that the platter spins at a fixed rate, but the sectors towards the outer edges of the platter are larger. By having larger sectors on the outer tracks, more data can be stored in the same angular space. The sectors become progressively smaller towards the inner tracks to maintain a constant rate of data passing under the read/write heads.

Higher areal densities allow drive manufacturers to pack more data onto each platter. Advancements in track density, sector size optimization, and read/write head technologies have enabled enormous growth in hard drive capacities over the years.

Constant Angular Velocity

Hard disk drives utilize a method known as constant angular velocity (CAV) to rotate the disk platters at a fixed rate. This means that the platters spin around at a constant speed, typically 5400, 7200, 10,000 or 15,000 rotations per minute (RPM). The rotational speed does not vary – it remains steady regardless of where the read/write heads are positioned.

Because the outer tracks of a platter are physically larger in circumference than the inner tracks, this means that when the disk rotates at a constant speed, the outer tracks move faster under the read/write heads. For example, if a disk spins at 7200 RPM, the linear velocity of the outermost track would be around 55 miles per hour. The innermost track would be moving significantly slower at around 24 miles per hour.

This difference in linear velocity is accounted for in the drive’s head positioning system. The constant angular spinning velocity combined with adjusting the head position allows data to be packed more densely on outer tracks. It also enables more data to be transferred per rotation on the faster moving outer tracks.





Hard disk drives have extremely tight tolerances to position the read/write heads over the tracks on the platter. The tracks are incredibly narrow, with widths measured in nanometers. For example, today’s high-capacity hard drives may have track widths around 70-100 nanometers.

To stay centered over these narrow tracks, the head positioning system must maintain tolerances well under 100 nanometers. Even the slightest vibration or shock can cause the head to drift off-track. Controlling the position of the head relative to the track is critical to ensure reliable reading and writing of data.

To maintain these tight position tolerances, hard drives use a closed-loop servo control system. This system uses position sensors to continuously monitor the location of the head over the track. If any deviation is detected, feedback control rapidly adjusts the positioning actuators to correct the head’s position and keep it centered over the track.[Modelling and control of a disk file head-positioning system](https://journals.sagepub.com/doi/pdf/10.1243/0959651011541300)

As tracks continue to narrow with higher data densities, even more precise position control will be required in the future. New actuators, sensors, and control algorithms are enabling sub-nanometer position tolerances to keep pace with the demands of growing storage capacity.

Future Improvements

Hard disk drive manufacturers continue to innovate and develop new technologies to increase the density and capacity of HDDs. Some of the emerging technologies include:

HAMR (Heat-Assisted Magnetic Recording) uses laser thermal assistance to enable higher density writing on high stability media. Seagate expects to release HAMR drives with capacities over 30TB by 2025 (1).

SMR (Shingled Magnetic Recording) increases density by overlapping tracks, like shingles on a roof. Western Digital uses SMR in some of their high capacity drives already (2).

Helium-filled HDDs replace air with helium to reduce turbulence and friction, allowing more platters to be packed into the same enclosure size. Both Seagate and Western Digital offer helium drives today (3).

Two-dimensional magnetic recording (TDMR) uses more powerful read/write heads to allow more bits within the same disk area. Companies like Seagate are actively developing this technology (1).

Together, these new techniques will enable HDD capacities over 100TB by 2025 and continue pushing the limits of mechanical storage density further (3). While SSDs are faster, HDDs will stay relevant where high capacity cheap storage is needed.

(1) https://www.securedatarecovery.com/blog/the-future-of-hard-drives
(2) https://guardiandatadestruction.com/resource-center/future-of-hard-disk-drives-industry/
(3) https://www.databarracks.com/future-of-processing-and-storage/storage.html


In summary, the physical construction of a hard disk platter enables the magnetic storage and retrieval of digital data. The concentric circles known as tracks provide a structure to organize the data, while sectors divide tracks into consistent storage blocks. The reading/writing heads can then precisely access data by moving inward or outward across the platter and switching between tracks. This method allows for densely packing data on the platter surface while still providing fast random access and reliability. The circular nature of the tracks and sectors and the precise control of the heads are critical to allowing hard disks to serve as performant mass storage devices in computers. As engineering improvements continue, the data density and performance of hard disks are increasing. But the fundamental magnetic recording and mechanical operation trace back to the ingenious physical layout of tracks and sectors on platters.