What are the two types of hard disks?

There are two main types of hard disks used in computers – HDD (hard disk drive) and SSD (solid state drive). The main differences between them are in how they store and access data.

HDD (Hard Disk Drive)

A hard disk drive (HDD) is a traditional storage device that uses magnetic storage to store and retrieve digital data. It uses one or more rotating platters coated with magnetic material to store data. A read/write head floats above the platter to read and write data. HDDs were the predominant form of disk storage for computers until the introduction of SSDs.

Some key characteristics of HDDs are:

  • Use magnetic storage to save data
  • Store data on spinning platters accessed by a moving read/write head
  • Relatively low cost per gigabyte compared to SSD
  • Slower read/write speeds than SSD
  • Vulnerable to damage from drops/shocks due to moving parts
  • Heavier and bigger in physical size compared to SSD
  • Higher latency and seek times for data access
  • Generate more heat and noise compared to SSD

HDDs have been the traditional hard drive technology for many decades. They offer large storage capacities at relatively low cost. HDDs are found as primary storage in desktop PCs and laptops, as well as enterprise storage servers. Though SSDs are replacing them in some markets, HDDs continue to be popular where large storage capacity is required at low cost.

How HDDs Work

Here is a more in-depth look at how HDDs work:

  • The hard drive enclosure contains one or more platters made of non-magnetic material, usually aluminum or glass. These platters are coated with a thin layer of magnetic material.
  • The platters spin at very high speeds, typically 5400 RPM to 15000 RPM for consumer HDDs.
  • A read-write head floats just above each platter surface on an actuator arm. It does not physically touch the platter.
  • As the platters spin, the heads can access data on any location on the drive. The heads move in and out across the platters to access different tracks.
  • The spindle motor spins the platters and the actuator motor controls the head arm movement.
  • Magnetized areas on the platter surfaces correspond to 1s and non-magnetized areas to 0s, storing data.
  • The heads contain electromagnets to read data bits from the platter surface by detecting changes in magnetic fields.
  • To write data, they contain small coils that generate magnetic fields to align magnetic particles on the platter surface.
  • A logic board contains controller circuitry to coordinate the I/O operations.

This electromechanical design makes HDDs capable of storing large amounts of data at an affordable price point. However, the reliance on physical moving parts also makes them relatively slow and prone to damage from shocks.

HDD Interfaces

Some common HDD interface types and standards include:

  • PATA – Parallel ATA, the earliest HDD interface, used a parallel bus for data transfer.
  • SATA – Serial ATA uses serial signaling instead of parallel. SATA has reached speeds up to 6 Gbit/s.
  • SAS – Serial Attached SCSI, a serial version of SCSI interface up to 12 Gbit/s.
  • FC – Fibre Channel, used for high speed SAN storage networks.
  • USB – Universal Serial Bus, HDDs are external portable storage devices.
  • NVMe – Non-Volatile Memory Express, a high performance protocol designed for SSDs but also used on HDDs.

SATA and SAS are the most common HDD interfaces in desktop and enterprise environments respectively. As speed requirements increase, NVMe is seeing wider adoption including with HDDs.

SSD (Solid State Drive)

A solid state drive (SSD) is a storage device that uses integrated circuit assemblies to store data persistently, typically using flash memory. Unlike electromechanical HDDs, SSDs have no moving mechanical components and instead use microchips to digitally store data.

Some key characteristics of SSDs are:

  • Use integrated circuits to store data (typically flash memory)
  • No moving parts unlike HDD
  • Much faster read/write speeds compared to HDD
  • Lower latency and access times than HDD
  • More resistant to physical shocks with no moving parts
  • Lighter and smaller form factors available compared to HDD
  • Significantly higher $/GB cost than HDD currently
  • Wear out over time with finite erase/write cycles per memory cell

The performance benefits of SSDs over HDDs have made them the preferred storage technology for consumer devices like laptops. Their adoption has also accelerated in enterprise storage. However, the higher cost per gigabyte still has HDDs leading in applications optimized for high capacity bulk storage.

How SSDs Work

Here is a more detailed look at how SSDs work:

  • The SSD controller connects to the host interface such as SATA or NVMe.
  • NAND flash memory chips store data in an array of transistor cells. Each cell traps electrons to represent a 1 or 0 bit value.
  • The cells are grouped in pages (e.g. 4KB). Pages are grouped into blocks (e.g. 256 pages/block).
  • Reads and writes occur at the page level. Erases happen at the block level, where an entire block of cells is reset to 1s.
  • The controller performs wear leveling to spread writes across all blocks evenly.
  • Error correction codes and checksums detect and recover from bit errors.
  • DRAM cache buffers improve write speeds and endurance.
  • Capacitors provide power loss data protection to flush caches.

SSDs provide fast, reliable storage by using flash memory instead of mechanical parts. However, there are some unique technical characteristics like write amplification and wear leveling that are handled by the SSD controller and firmware.

SSD Form Factors

SSDs are available in various physical form factors, capacities, and interfaces. Some examples include:

  • 2.5″ SATA SSD – Common in laptops and desktops.
  • M.2 NVMe SSD – Compact footprint for ultrabooks and tablets.
  • PCIe Add-In Card – High performance expansion card SSD.
  • 1.8″ or smaller micro SATA – Ultra compact SSDs for embedded and mobile devices.
  • U.2 – Enterprise form factor to fit in HDD slots.
  • Ruler – Specialized form factors for data center storage designed for density.

The smaller physical size of SSDs enables innovative form factors. However, 2.5″ and M.2 remain the most popular. Enterprise SSDs use more exotic designs for high density in racks.

Comparison Between HDD and SSD

Here is a summary comparing the major differences between hard disk drives (HDD) and solid state drives (SSD):

Storage medium Magnetic platters NAND flash memory
Moving parts Yes (platters, head, spindle) No
Shock resistance Low due to moving parts High with no moving parts
Read/write speeds Lower, mechanical latency Higher, no physical seeks
Noise Audible spinning and seeks Silent
Power consumption Higher, must spin platter Lower when idle
Heat output Higher due to moving parts Lower
Size and weight Larger and heavier Smaller with lighter options
Reliability MTBF lower Higher MTBF
Data retention Indefinite magnetic storage Finite write endurance
Cost per GB Much lower Higher currently

In summary, SSDs provide significant performance, reliability, weight, and acoustic benefits over HDDs. However, HDDs continue to offer a better price per gigabyte of storage making them preferable for high capacity bulk storage needs.

Use Cases for HDDs vs SSDs

There are certain applications where HDDs continue to be preferred over SSDs:

  • Data archiving and backup – The low cost per gigabyte makes HDDs preferable for bulk storage and infrequently accessed data.
  • Media libraries – Large media libraries with videos, photos benefit from HDD’s higher capacities.
  • Gaming consoles – The higher capacities allow storing more games locally.
  • NAS/RAID storage – Network attached storage and RAID benefit from HDD’s capacity/cost.
  • Surveillance storage – Constant video recording favors HDD’s capacity and sequential writes.

SSDs tend to provide more benefits for the following use cases:

  • OS drive – The SSD speed boosts boot times and overall system performance.
  • Frequently used programs and games – Quicker load times and level changes.
  • Tablets and notebooks – Improves responsiveness and battery life.
  • Mission critical enterprise apps – Lower latency helps demanding applications.
  • Noise sensitive environments – Silent operation without moving parts.

Overall, HDDs are preferable where high capacity bulk storage is required. SSDs provide better performance especially for more responsive computing applications.


There are two main types of hard drives – the traditional HDD that uses spinning magnetic platters, and the newer SSD that uses flash memory chips. HDDs are able to provide much higher storage capacity at a lower cost. However, SSDs offer significant benefits in performance, reliability, power efficiency, weight, and acoustic noise. While HDDs still have a place for high capacity storage, SSDs are the preferred technology for applications requiring high speed, low latency storage. Moving forward, expect to see SSD capacities continue to increase while costs decrease, allowing them to replace HDDs in more and more use cases.