What is a hard disk drive (HDD)?

The hard drive is an important component that is used to store the computer data and programs as well as the operating system. The term hard drive comes from the 1970s, when IBM introduced the first “Winchester” drives in which the read/write heads and platters came together in a sealed unit, therefore called “hard”. These first hard drives only had a capacity of a few tens of megabytes (MB) while in 2021 some consumer hard drive models exceed a capacity of 10 terabytes (TB).

Seagate 16TB HDD

There are currently two main technologies of hard disks with very different internals: mechanical hard disk drives (HDD) and solid-state disks (SSD). The HHDs use magnetic platters which rotate very quickly (several thousand revolutions per minute) and heads which read or write data, these are the traditional hard disks used these last decades. The SSDs are far more recent, they have no mechanical parts and use memories whose content is preserved even when the computer is turned off and the disk is not supplied with electricity.

Solid-state disks (SSD) are faster than mechanical hard drives (HDD) but their storage capacity is lower for the same price (their storage capacity is nevertheless increasing steadily and models of several terabytes can be found in 2021). The price of SSDs is currently higher than that of HDDs for the same capacity but if it is steadily decreasing.

Samsung 980 Pro SSD

What are the different hard disk drives form factors?

The most common form factors for hard disk drives in 2021 are:

  • 3.5-inch form factor for desktop computers. This large form factor allows the most important capacities for consumer hard drives.
  • 2.5-inch form factor used by mechanical hard drives (HDD) in laptops and by many external drives. The capacities and performance of these drives are generally lower than those of 3.5-inch drives because their platters are smaller and they spin less quickly. Some solid-state drives (SSD) also use this form factor, which allows them to easily replace mechanical drives to improve the computer performance.
  • m.2 2280 form factor used by solid-state hard disks (SSD) with dimensions of 22 mm x 80 mm.
  • m.2 2242 form factor with a smaller size used by solid-state hard drives (SSDs) with dimensions of 22 mm x 42 mm.

What are the main characteristics of a hard drive?

  • Internal or external: the former are installed inside the computer or laptop case while the latter are in an external case.
  • Technology used: in 2021, hard drives are either mechanical (HDD) with magnetic platters, or solid-state (SSD) with flash memory. Some hard drives are hybrid and use both technologies at the same time.
  • Connection to the computer: Consumer internal hard drives use either a SATA connector or an M.2 connector to connect the hard drive to the motherboard. The former is used by mechanical disks (HDD) and some solid-state drives (SSD), while the latter is used only by solid-state drives (SSD). External hard drives usually use a USB cable for their connection to the computer, some use a thunderbolt cable.
  • Storage capacity: this is what determines the amount of data or programs that can be stored in the hard drive. In 2021, it ranges from a few tens of gigabytes (GB) for the smallest ones to more than 10 terabytes (TB) for the “biggest” consumer hard disks.
  • Transfer rate: the higher it is, the less time it will take to read or write data from/to the hard drive. It is often measured in megabytes per second (MB/s) and it ranges from a few megabytes per second to over 100 MB/s for mechanical disks (HDD) and can reach several gigabytes per second (GB/s) for the fastest solid-state drives (SSDs).
  • Response time: the lower it is, the faster the disk will access data. Mechanical hard drives (HDD) have far slower response time than solid-state drives (SSD), it is in the order of milliseconds for mechanical hard drives while solid-state drive (SSD) have a response time in the order of tens of microseconds or less.
  • Number of operations per second (IOPS): the higher this number, the faster the hard drive will be when several programs (including the operating system) need to access data on the hard drive at the same time. Solid-state drives (SSD) perform again much better than mechanical hard drives (HDD) here. The former can exceed hundreds of thousands of operations per second while the latter will rarely exceed hundreds of operations per second.
  • Communication interface with the motherboard used by internal drives: in 2021, mechanical drives generally use the SATA 6.0 Gb/s interface while solid-state drives (SSD) use either this same interface or the faster PCI-Express interface.

Price and choice of hard drive:

The price of a hard drive will depend primarily on technology and capacity. Mechanical hard disk drives (HDD) are less expensive than solid-state drives (SSD) for the same capacity, but they are also slower. Desktop computers often allow to connect multiple internal hard drives. This will allow you to use a fast solid-state drive (SSD) for the operating system and programs, and one or more mechanical drives (HDD) for the rest (photos, music, videos or backups for example).

However, it is not always possible to install more than one hard drive in a laptop computer and it is sometimes even soldered directly to the motherboard and cannot be changed. It will therefore be necessary to choose the technology and the size of the internal hard drive depending to your needs. We recommend you to use a solid state drive (SSD) in this case, they are lighter and faster than mechanical drives. And in case of need, it is possible to use an external hard drive for backups or to store large files.

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