How A Hard Drive Works

Learning how a hard drive works can give you a solid foundation when you decide to build your own computer or to do a simple upgrade. When you know how a hard drive works, shopping for a new drive will be easier as you will know what to look for in a drive. You will also understand what the geek at the computer store is talking about when he starts spouting off numbers and abbreviations to you.

This article will take a look at how a hard drive works. It will explain platters and spindles, actuators and heads. It will also take a look at how hard drives actually store and retrieve data. I will cover capacities, and read/write times. This will give you a solid foundation on what to look for when you are planning to purchase a new hard drive.

Hard Drives

A hard drive, also known as a hard disk drive (HDD) or disk drive, is a non-volatile memory device, meaning that it will retain its information even when no power is present. This is accomplished through digitally storing the data on rapidly spinning platters with a magnetic surface. 

This is how a hard drive works. The data storage medium, the platter, is a circular disc which has a magnetic surface that actually stores the data.  Hard drives will typically have multiple platters on which to store data. These platters are usually made out of aluminum or glass substrate. A thin coating is applied to each side of the hard drive during manufacturing to apply the magnetic surface. That is covered by a carbon based overcoat, and on top of that is a extremely thin layer of lubricant. The surface is then tested for smoothness and balance by what is called a “flying head”.

The flying head is what actually reads and writes the data on the platters. When the platters spin, the read/write head will “fly” over the top of the platter at a distance of about 3 nanometers, or 3 billionths of a meter. The height of the head is controlled by a combination of an air-bearing, and the spindle.  The spindle relies on air pressure inside the hard drive to maintain that height. Should the head drop too low and touch the platter, you will get what is called a head crash. 

Head crashes can occur from sudden power loss, physical shock such as dropping the drive, wear and tear over long periods of time, or poorly manufactured platters. Head crashes are not a good thing. They can cause data loss by grinding away the thin magnetic film that covers the surface of the platter. If you start having problems with a particular application or your operating system, and you recently kicked or dropped your computer, this may be the cause.

A hard drive head is wrapped in a wire coil. When data needs to be written to the platter, the coil will energize and a strong magnetic field will form in the middle. The surface of the platter underneath the head will then magnetize. When it needs to read data from the platter, the magnetized area on the platter passes by the head. The magnetized area concentrates the field, and energizes the coil. Keep in mind that this entire process is happening extremely fast.  On average, a hard drive’s platters spin at 7200 RPM, or 120 times per second.

Now that you know how a hard drive works, lets take a look at two other topics that will further your knowledge of how a hard drive works.

Hard Drive Capacity

To figure out the capacity of a hard drive, you multiply the number of cylinders by the number of heads by the number of sectors, by the number of bytes/sector (usually 512). Or, you just look at the label and see that it has a capacity of 120GB. Of course, you have probably noticed that in Windows, you see a different size reported for the size of your drive. There is a good reason for this.

Hard drive manufacturers and your operating system use two different forms of measurement, and both happen to be correct. Windows uses binary measurement, and the manufacturer uses decimal measurement. To see the differences here, we have to look at binary numbers compared to decimal numbers. Remember that binary numbers are a power of 2, and decimal a power of 10.

• The binary number 2^10 = 1024, and the closest decimal equivalent is 3^10, or 1000.
• The binary number 2^20 = 1,048,576, and the closest decimal equivalent is 10^6, or 1,000,000.
• The binary number 2^30 = 1,073,741,824, and the closest decimal equivalent is 10^9, or 1,000,000,000.
• The binary number 2^40 = 1,099,511,627,776 and the closest decimal equivalent is 10^12 or 1,000,000,000,000.

Now, look also at the prefixes used to describe data sizes.

• Kilo = 1 Thousand
• Mega = 1 Million
• Giga = 1 Billion
• Tera = 1 Trillion

So if we take these numbers and divide them we get the following:

• 1000/1024 = .977
• 1,000,000/1,048,576 = .954
• 1,000,000,000/1,073,741,824 = .931
• 1,000,000,000,000 / 1,099,511,627,776 = .910

This is one of the quirks with how a hard drive works. As you can see, the more data you are measuring, the greater the discrepancy.  When you get to the Terabyte level, as the latest drives are, you get a discrepancy of 9%. That’s going to be quite a difference. So to be correct here, Windows is actually reporting the incorrect size. Windows uses binary calculations, and then puts the GB next to it, when technically that is not correct due to the definition of Giga.  Although this confuses things, don’t expect the manufactures to change, and do you think Windows will change? But on the flip side, would you rather see the drive sold as a 2^40 byte drive, or a 1 Terabyte drive? Just keep in mind that the drive you buy will be the advertised size, Windows just shows it differently.

Hard Drive Speed

Finally, let’s take a look at read and write speeds. The speed of the hard drive is reported RPM’s, or revolutions per minute. This is a measure of the speed that the platter is spinning at. The faster the revolution, the more data passes beneath the read/write head. The faster it spins, the more data can be delivered or written within a specific timeframe. 

Most desktop hard drives today spin at 7,200 RPM, and the average laptop hard drive spins at 5,400 RPM. 10,000 RPM drives are available for desktops from Western Digital, but they are going to cost significantly more. You can get a 500GB 7,200 RPM drive for the same price as a 150GB 10,000 RPM drive.  A laptop hard drive is also available at 7,200 speeds, but because they consume more power, they are not the norm. Depending on your use of the laptop, when you purchase be sure to check the drive speeds. If you are doing any gaming or software development, you will want to go with the 7,200 speed drive. Also keep in mind, that even when you are purchasing a high end laptop, you should check the drive speed as the default for almost all laptops is 5,400 RPM drives.

If you are interested in learning how to install a hard drive, check out this article.

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