What are the Four Major Components of a Hard Drive (HDD)?

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Hard Disk Drive (HDD)

If you have owned a computer in the last fifteen years, chances are extremely good that you have owned a Hard Disk Drive, abbreviated in the tech world to HDD. When you take a photo and save it to your computer or download some music or a game from the Internet, you probably don’t even think about how that data is stored. Just click ‘save’ and your computer does all the work. On the surface, it’s instantaneous and simple, but humming inside your computer case is a delicate piece of equipment working hard to protect your data.

The Hard Drive is made up of four major components, all working together to create the storage devices we so often take for granted.

Drive Heads (or Head Stack Assembly)

The first and probably the most important part of any HDD is the drive head. Many people remember having to clean the heads on their VCR or cassette deck. The drive heads are sensitive, precision instruments and are generally the most frequent cause of hard drive malfunction. This electronic piece makes possible the actual reading and writing of data, by making minuscule and extremely fast movements to precise points on the platter, like the needle of a record player, where data is stored and retrieved through a complex system of magnetism and electric currents. The drive heads are held by an acutator arm just about a micron (a very small distance) away from the platter. Be careful! If the heads were to come in contact with the platter, for example, or if the drive were to be dropped or significantly jarred, it could cause massive damage and often irrecoverable data loss.


Just as it sounds, the spindle is fairly self-explanatory. In order for the drive head and actuator arm to be able to utilize the disk’s entire surface area, the platter must be spun. With growing file sizes and transfer speed such an important part of computing, the platter must spin extremely fast. Most drives today are considered “high-speed” and reach average speeds of 4800 to 7200 rotations per minute. The more rotations per minute, the faster the drive, meaning less time reading and writing data. The fairly recent adoption of SSD or Solid State Drives has further increased read/write speeds by creating a drive with few or no moving parts, using silicon as the medium for data transfer, as opposed to a magnetized platter such as those described here.


The sturdy metal enclosure around a hard disk drive is intended to protect the hard drive’s assembly, but most noteably to protect the highly sensitive platter where all the magnetic data is stored. With so much precision and minuscule measurements of magnetism and data, even the tiniest abnormality on the platter can render a drive entirely useless. The platters have highly specific placement and in some drives which have multiple platters, even the most minuscule misalignment could prevent a drive from functioning properly. Fortunately, drives are constructed with such sensitivity in mind and have on occasion been known to withstand more punishment than perhaps they ought to.

Electronic Circuitry

On the back side of an internal hard drive (or somewhere inside the enclosure of an external HDD) is a piece of green circuit board with all the electronic resistors, capicitors, and other bits and pieces that hold it all together. All the other parts are connected to the circuit board, which contains basic electronic instruction known as firmware, which tells it all just exactly how to work. It is into the circuit board that you plug your power cable and data transfer cable (often SATA, PATA, or on some older systems, IDE and UltraIDE). The circuit board receives electronic impulses from the rest of the computer and sends the appropriate instructions to the spindle and the drive head, which all work in perfect tandem to find the right data.

Clearly, there is more to the specifics of specifically how data is stored, read, and altered by magnetism and electricity, as well as the minute construction and precision of a hard drive’s manufacturing. However, understanding these basics is the first step in a larger understanding.