A hard disk drive, commonly referred to as a hard drive, is a data storage device used in computers to store and retrieve digital data. Hard drives have rigid platters coated with magnetic material inside protective casings. Data is written to and read from the platters using read/write heads that float just above their surfaces. Hard drives come in a variety of shapes and sizes depending on their form factors but the core components and functionality remain essentially the same.
What is a Hard Drive?
A hard drive is a non-volatile data storage device, meaning it retains data even when powered off. It houses one or more platters made of rigid non-magnetic material, generally aluminum or glass, that are coated with a thin layer of magnetic material. Platters are arranged in a stack and rotate at high speeds of typically 5,400, 7,200, 10,000 or 15,000 rpm nowadays. The platter surfaces are divided into tracks which are further subdivided into sectors for storing data. Read/write heads on actuator arms hover over the platter surfaces to access data. As platters spin rapidly, the heads can access data across the surfaces in an extremely fast manner.
Internal Hard Drive Form Factors
Internal hard drives for desktop computers typically conform to form factors standardized by industry associations. Common form factors include:
3.5-inch
The 3.5-inch hard drive is the most common internal hard drive form factor. They are designed to fit in 3.5-inch drive bays and connect to motherboards via SATA interfaces. 3.5-inch hard drives typically offer the largest storage capacities compared to other form factors, with most consumer models ranging from 500 GB to 10 TB. The platters are stacked vertically and enclosed in a squared metal case measuring approximately 4 x 6 x 1 inches.
2.5-inch
2.5-inch hard drives are smaller than 3.5-inch drives and designed for laptops and compact devices. They utilize smaller platters arranged in a horizontal orientation and fit into a rectangular aluminum or stainless steel housing measuring approximately 2.75 x 3.96 x 0.27 inches. 2.5-inch drives connect via the SATA interface and are also commonly used in external hard drive enclosures. Storage capacities typically range from 250 GB to 2 TB for consumer HDDs.
1.8-inch
The 1.8-inch hard drive has a very compact form factor designed for small mobile devices. The microdrive utilizes tiny 1.8-inch platters stacked vertically in a stainless steel or aluminum enclosure of approximately 1.7 x 1.4 x 0.2 inches. Storage capacities of 1.8-inch hard drives range from 4 GB to 128 GB. This form factor has declined in usage with the rise of SSDs in mobile devices.
mSATA
mSATA or mini-SATA drives are designed for tablet computers and ultrabooks. The form factor measures approximately 1.4 x 1.2 x 0.2 inches. mSATA solid-state drives fit onto a circuit board and do not require trays or enclosures. Interface is SATA-2 or SATA-3 compatible. Capacities range from 32 GB to 1 TB. mSATA is declining in favor of M.2 solid-state drives.
External Hard Drive Form Factors
External hard drives are encased in protective enclosures and utilize common desktop hard drive inside. Common external hard drive form factors include:
3.5-inch
These house a 3.5-inch hard drive inside an enclosure that provides ports to connect to the computer. The enclosure is slightly larger than a bare 3.5-inch hard drive and includes ventilation holes. A fan may be included for active cooling. External power brick is required.
2.5-inch
Much smaller and lightweight drives utilizing 2.5-inch hard drives inside. Often powered through USB so no external power supply needed. Durable aluminum is commonly used for the enclosure. Provides portability due to small size.
1.8-inch
Smallest external hard drives with 1.8-inch microdrives inside plastic or aluminum enclosures. Very compact and portable.
Hard Drive Components
While hard drive form factors may differ, the core internal components and how they work remain similar across the various drive types.
Platters
Platters are the disks made from non-magnetic material, generally aluminum or glass, that are coated with a thin magnetic layer for storing data. Most hard drives have multiple platters stacked on top of each other and separated by spacer rings. Platters spin rapidly during drive operation. The platters are housed within the drive chassis.
Read/Write Heads
Also known as read/write transducers or heads, these are electromagnetic devices on actuator arms that move over the drive platters to read and write data. The aerodynamically designed heads float nanometers above the platter surface on an air bearing generated by the spinning disks.
Spindle Motor
This electric motor precisely spins the hard drive platters at speeds up to 15,000 rpm in modern drives. The spindle motor assembly also houses the spindle which physically connects the platters and allows them to spin as a single unit.
Actuator Arm and Actuator
The actuator arms hold the read/write head assemblies over the platters surfaces. They move the heads from the inner tracks to the outer tracks across the platters for accessing data. The actuator is a linear or rotary motor that swings the arms to accurately position the heads. Voice coil actuators that use magnets and coils are commonly used.
Drive Controller
The drive controller or control board houses the drive’s main controller, spindle driver and motor driver. It provides the interface for the drive to connect to the computer’s motherboard and power supply. The controller governs all the electronic and mechanical operations within the drive.
Casing
The protective metal or alloy housing that seals and contains all the drive’s components. It includes holes for ventilation and mounting.
How a Hard Drive Works
Hard drives work on electromechanical principles to magnetically read and write digital data. Here are the key processes explaining how they work:
Writing Data
1. The write head converts an electrical signal into a magnetic field which magnetizes a tiny section of the platter’s magnetic layer, thereby writing a binary 0 or 1.
2. The platter rotates allowing billions of these tiny magnetized sections representing 0s and 1s to be written in narrow circular tracks on the platter surfaces.
Reading Data
1. As the platters spin, the read head hovers over the tracks. The magnetic fields from the magnetized sections on the platter surface induce a voltage in the read head.
2. This voltage is converted into the original 0 and 1 binary data by the drive electronics.
Finding Data
1. The drive controller moves the actuator arms to position the heads over the required track where the desired data is located.
2. The heads only read data from tracks directly under them as the platters spin.
3. The actuator arms race back and forth rapidly to access data from different tracks.
Evolution of Hard Drive Technology
Hard drives have undergone major improvements in technology over decades while retaining the same essential principles of operation.
1950s – First HDDs
IBM released the first hard drive, RAMAC 305, in 1956. It used fifty 24-inch platters and could store 5 MB of data. Massive in size, it was a technological marvel then.
1970s – New HDD Technologies
Many innovations in the 1970s improved HDD capacity and performance:
– 8-inch and 5.25-inch form factors with aluminum platters
– FM encoding for increased storage density
– MFM encoding for further density gains
– Thin film heads for improved read/write capabilities
– Closed loop servo control for accurate head positioning
1980s – Miniaturization and High Capacity
Hard drives became smaller yet higher capacity through the 1980s:
– 3.5-inch and 2.5-inch smaller form factors
– MR heads enabled further increases in areal density
– Spindle motors rotated platters faster – up to 3600 RPM
– Voice coil actuators fine-tuned head positioning
– IDE/ATA interfaces for easy installation
– Storage capacities grew from 10s of MBs to 100s of MBs
1990s – GMR heads and PRML read channels
Giant magnetoresistive (GMR) heads drove new heights in HDD capacity. Partial-response maximum-likelihood (PRML) read channels provided more accurate data recovery. Areal densities jumped from <100 Gb/in2 at the beginning of 1990s decade to over 1 Tb/in2 by 1999.
2000s – Perpendicular Recording and Larger Capacities
Perpendicular magnetic recording enabled further density increases leading to capacities of multiple terabytes by 2009. 2.5-inch mobile form factor drives became predominant as desktop sizes shrunk. SATA interfaces replaced IDE providing faster data transfer speeds.
2010s – Shingled Magnetic Recording
Shingled magnetic recording (SMR) technology allowed squeezing more tracks onto platters by overlapping tracks. Storage capacities crossed into 10 Tb+ territory by the late 2010s for both enterprise and consumer drives. Solid state drives (SSDs) also emerged as an alternative.
2020s – Heat-Assisted Magnetic Recording
Heat-assisted magnetic recording (HAMR) and microwave-assisted magnetic recording (MAMR) will likely enable drives with 40 Tb+ capacities. However, SSDs are taking over most desktop and mobile storage applications, relegating HDDs primarily for bulk data storage. The HDD industry will continue innovating and improving platter areal densities and capacities along with reliability and performance.
Conclusion
While physically larger and slower than SSDs, hard disk drives remain highly relevant for large-scale, high-capacity data storage purposes. Under the hood, modern hard drives still work much the same way that the pioneering IBM RAMAC drive did back in 1956. But monumental advances in technologies like heads, media, and controllers have enabled stunning gains from megabytes to terabytes in capacities over decades while the core electromechanical architecture remains. Hard drives will likely continue evolving and adapting for the foreseeable future as a cost-effective storage workhorse technology.