Hard disk drives (HDDs) have been the predominant form of computer data storage since the 1960s. An HDD is a non-volatile storage device, meaning it retains data even when powered off. Hard disk drives consist of one or more rigid platters coated with a magnetic material, along with read/write heads that move across the platters to access data. In this article, we will look at the physical components inside a hard drive that allow it to store data. We will examine how binary data is encoded onto the magnetic platters, and how the drive’s file system organizes this raw data into files and folders. Understanding the inner workings of a hard drive provides insight into an essential computer component that has persisted for over half a century.
Hard Drive Components
A hard drive consists of several key components that work together to store and retrieve data. The main components include:
Platters – The platters are disk-shaped plates made of a non-magnetic material, typically aluminum or glass, that are coated with a thin layer of magnetic material. Data is stored on the platters in the form of magnetic polarities. The platter rotates at high speeds on a spindle while the read/write heads access different areas of the platter to read or write data. Hard drives contain either one or multiple platters stacked on top of each other to increase storage capacity [1].
Read/Write Heads – The read/write heads are electromagnetic devices that float just above the surface of the platters on an air bearing. They read and write data on the platters by magnetizing or sensing the magnetism of the material coating the platters. There is one read/write head per platter surface [2].
Spindle – The spindle is a rod that spins the platters at very high speeds, typically 5,400 to 15,000 rpm. The rotational speed of the spindle is a key factor affecting the overall speed and performance of the hard drive.
Actuator – The actuator is a component that moves the read/write heads to the desired track and sector across the platters during data access operations. It contains a motor that provides the force to move the heads.
Together, these components allow data to be stored in magnetic form and rapidly accessed from anywhere on the hard drive platters.
Platters
Hard disk drive platters are the circular disks inside the drive that actually store the data. Platters are made from either aluminum, glass, or ceramic and are coated with a thin magnetic film layer. This magnetic coating allows data to be stored on the platters magnetically in small sections called tracks and sectors (Hard disk drive platter).
The platters rotate at very high speeds up to 15,000 rpm in the hard drive. Read/write heads float nanometers above the platter surface, reading and writing data to the magnetic coating as the platters spin. The data is encoded and decoded into magnetic patterns by the read/write heads.
In contrast to traditional hard disk drives (HDDs) that store data on magnetic platters, solid state drives (SSDs) use flash memory chips and have no moving parts. However, HDDs allow for greater capacities at lower costs compared to SSDs. The platters and magnetic storage method are what allow HDDs to provide large storage capabilities at affordable prices (Are Hard Drive Platters Worth Anything?).
Read/Write Heads
The read/write heads are responsible for reading and writing data on the platters. They are located at the end of an actuator arm and float just above the surface of the platters on a cushion of air as the platters spin at high speed. The heads contain electromagnets that generate a magnetic field to read or modify the magnetic orientation of tiny areas on the platter surface. Each head is connected to the disk controller, which moves the heads to the desired track and sends or receives the data.
To read data, the heads detect the magnetic orientation of the bits on the platter and convert that into electrical signals representing 0s and 1s. To write data, they pass an electric current through the electromagnet to orient the magnetic polarity of the bits into a pattern representing the data. The heads must be precisely aligned over the track to read or write the data correctly. As platters stack on top of each other, separate heads are required for the top and bottom surface of each platter. The collection of heads is called the head stack.
The read/write heads are extremely delicate and are designed to “fly” just above the platter surface without touching it, typically around 3-10 nanometers above. If the head touches or crashes into the platter, it can destroy data in that area. To prevent this, modern hard drives park the heads in a safe position when not in use.
Sources:
https://en.wikipedia.org/wiki/Disk_read-and-write_head
https://www.techopedia.com/definition/9112/readwrite-head
Tracks and Sectors
The hard disk platters are divided into concentric circles called tracks. Tracks are located on both the upper and lower surfaces of each platter. The tracks are divided radially based on their distance from the center of the platter. Tracks that are closer to the center of the platter have a smaller circumference than tracks toward the edge.
Each track is further divided into sectors which are pie-shaped wedges radiating outward from the center of the platter. A sector is the smallest physical storage unit on a hard drive. Common sector sizes are 512 bytes, 4,096 bytes, or 4,294,967,296 bytes. The number of sectors per track depends on the track’s location. Tracks closer to the center have fewer sectors due to their smaller circumference while tracks toward the edge can contain more sectors. This allows for efficient use of the full platter surface to maximize storage capacity.
This division of the platters into tracks and sectors provides an organizational framework to locate and access data. The drive’s read/write head can move to a specific track then read or write data to the sectors within that track. The location of each sector is defined using the cylinder number, head number, and sector number, also known as CHS addressing. The granular ability to individually access sectors for reading and writing enables efficient storage and retrieval of data on the hard drive.
Reference: https://www.open.edu/openlearn/digital-computing/introducing-computing-and-it/content-section-5.3
Cylinders
A cylinder is comprised of all the tracks on multiple platters at the same head position. As the platters rotate together on the spindle, the read/write heads move in unison across the surfaces of the platters, accessing tracks that make up a cylinder. This coordinated movement allows efficient sequential access to all the tracks within a cylinder without any seek latency delays. So when data is written sequentially, it will be written cylinder by cylinder across the platters. The total number of cylinders depends on the number of tracks per platter and number of platters in the hard drive.
For example, a hard drive with two platters that each have 1000 tracks per surface would have 2000 tracks in a cylinder (4 surfaces x 500 tracks per surface). The cylinders are then accessed by the actuator arm moving the read/write heads in tandem.
Accessing Data
In order to access data stored on a hard drive, the read/write head moves between the disk platters to hover above the tracks and sectors where data is located. The hard drive uses a movable arm called an actuator that allows the head to move back and forth across the platters. Electro-motors carefully control the actuator’s movement to position the head over the exact cylinder and track where the desired data resides. The hard drive controller receives instructions for where data is located on the platters from the file system. It then moves the actuator arm to move the head to the proper cylinder and rotates the platter so the requested track lines up with the head. The head can then read or write data in that track and sector. Once the operation is complete, the actuator arm rapidly repositions the head to access data in another area of the platters. The incredible speed and precision of head movement allows data to be located and accessed nearly instantaneously.
The hard drive relies on this intricate choreography of head positioning to enable lightning fast random access data storage and retrieval across the surface of the platters. Without the actuator’s ability to swiftly move the head to any cylinder or track, locating specific data would require much more time and effort. This key innovation transformed hard drive technology by allowing efficient direct access to any piece of data based on its unique location.
Data Encoding
When storing data on a hard drive, the information needs to be encoded in a format that can be written and read magnetically using zeros and ones. One of the most common encoding schemes is called modified frequency modulation (MFM) [1]. In MFM, binary data is encoded using magnetic transitions, which represent a change between a north and south pole on the platter’s magnetic surface. A binary 1 is represented by a magnetic flux transition, while a binary 0 is represented by no transition [2].
So a sequence of 1’s and 0’s in binary data would be converted to a pattern of magnetic transitions and non-transitions as the write head moves across the platter. The read head can then detect these magnetic flux changes and decode them back into the original binary data. This allows massive amounts of digital data to be efficiently encoded magnetically as a series of magnetic poles on the platter’s surface represented by zeros and ones.
File System
The file system is responsible for organizing data into files and folders on a storage device like a hard drive. It provides a way to name, access and organize files so the operating system and applications can find data when needed. Some common file systems for hard drives include NTFS for Windows, HFS+ for Mac, and exFAT for external drives. NTFS uses advanced data structures to enhance performance, reliability and disk space use. It organizes files into hierarchical folders, assigns access rights and supports large partition sizes. The file system maintains metadata like filename, size, location, and date modified to manage files effectively. The hard drive relies on the file system to provide logical storage and retrieval of data for the operating system.
Conclusion
In summary, the physical location of data inside a hard drive is on the platters which are made of non-magnetic material and are coated with a thin layer of magnetic material. Data is stored in the form of magnetized and demagnetized spots on the platters. The read/write heads float above the platters on an air cushion and can detect and change the magnetic orientation of these spots to read or write data. The platters rotate rapidly and the heads move across the platters in arcs across the tracks and sectors which make up the hard drive’s cylinders. Through this process, the hard drive is able to store and retrieve digital data by accessing precise locations on the platters.
The ingenious engineering of hard drives allows for the safe, dense, and rapid storage of vast amounts of digital data that computer systems rely on. Though newer technologies like SSDs offer advantages, the principles behind hard drive data storage continue to evolve and serve as the foundation for many computing applications.