A Brief Introduction to
Hard Drive Technology

Hard drives are a marvel of technology. Their basic operation is simple and they appear to be "black boxes" which are used to store and retrieve data. However, an elementary understanding of the inner workings of these storage devices can be helpful in preserving your data.

Modern hard drives are considerably more reliable than the first PC disks of the eighties but their basic operation is still the same.

Cost and Size Benefits

The best story of hard drive technology is about how the disk cost has been reduced from $100 per MB in the early eighties to below $0.50 per GB today. The march of technology continues even today, such that the leading hard drives double in capacity every two to three years.

Defining Features of The Hard Disk Drive

The simplicity of the hard disk drive lies in the few components that make up the drive. The drive case with its controller card, the magnetically coated platters, the actuator arm, and the read/write heads. Of these parts, the two moving components are the stack of platters and the actuator arm assembly. However, the complexity of this technology lies in the density of the data on the platters and in the ability of the heads to read and write that data.

Hard Drive Form Factors

The hard disc frame is typically made of aluminum. The size of the frame is determined by the form factor, which is the external dimension of the unit. Typical PC hard disk form factors are: 3.5", 2.5", PC Card (PCMCIA), and Compact Flash. The internal disks usually have the controller card mounted to the exterior side of the frame. The frame does not completely seal the inside from the outside environment. There is a filter keep out dust but air can still pass into and out of the unit.

Hard Drive Platters

Platters consist of a hard substrate covered with a thin coating of magnetic material. The data is stored on the magnetic surface in structures called tracks, which are concentric circles on the disc surface. The latest substrates are made of glass, or more specifically a ceramic composite with inserts. The magnetic surface is made from a coating called thin film which gives it a silver color. The drive contains multiple platters that are stacked on top of one another with just enough room between the platters for the read/write heads. Typically, data is stored on both sides of a platter. The platters are connected with a spindle that is attached to a motor.

The platters can be spinning anywhere from 3600 RPM to 15,000 RPM, depending on the drive type. The technological trend is to higher RPMs since it allows the data to be read more quickly. As drives start spinning faster, they create more heat and noise. This makes it important to monitor the hard drive temperature. The help reduce the drive temperature, you might want to purchase a hard drive cooler.

Another technique to reduce drive heat is make the platters smaller. A smaller platter can be spun up to higher RPMs. There is also the added benefit that the heads have less distance to travel. The smallest hard disk drive today is the microdrive. However, even though the microdrive is physically smaller, the larger form factors may also be constructed with smaller platters.

Actuator Arm

The head actuator assembly moves the read/write heads to specific locations on the platters. Each side of the platter has its own read/write head and all of the heads move in tandem. The actuator seeks locations on the platters called cylinders. A cylinder consists of all the tracks stacked on top of on another at a specific location. Only one platter surface (head) can be read or written at any particular moment.

Read/Write Heads

While the platters are spinning, the heads never actually touch the surface of the platters. They are literally flying over the surface at a distance measured in millionths of an inch. The height of the heads above the disk surface is much less the thickness of a human hair or even a dust particle. If the head were to touch the surface and scrape off some of the magnetic material, the drive would experience a head crash. Technology for the latest disk surfaces includes special lubricants and protective coatings to help avoid a hard drive crash, but it cannot prevent damage altogether. Because of this, it is important to avoid jolting or vibrating the hard drive while it is spinning.

Perpendicular Magnetic Recording (PMR)

The most significant advances in areal density in recent years come from the implementation of Perpendicular Magnetic Recording. This technology was first demonstrated in 1976 but it has only been in the last ten years that hard drive manufacturers have been using it in earnest. Before perpendicular recording, the bits were laid out entirely on the surface of the platter. However, with this new technique, individual bits are stored at various levels in the magnetic media. Sort of like different levels of an underground building.

The latest masterpiece of PMR is the terabyte hard drive.

Flash Drive Technology

The terms "hard drive" or "drive" are often used to simply designate an abstract storage device. Two examples of this trend are the solid state hard drive and the flash drive.

There are two types of flash memory: NAND flash and NOR flash. NOR flash is like SDRAM were each individual byte can be accessed directly, and NAND flash is more like a block access device (think hard drive) where the data is read and written in large blocks at a time. NOR is typically used for firmware and NAND is used for flash drives of all sorts.

Flash drives are starting to replace hard drives in some specialized applications such as mobile devices and rugged portable computers. However, NAND flash still has some enormous problems to overcome before it can completely replace the magnetic rotating platters we have come to know as the hard disk drive.

The most obvious hurdle is the cost per GB but this is a small hurdle for a disruptive technology that has so many benefits, foremost of which are its resistance to vibration and faster access times. The bigger hurdle has to do with the larger block sizes and the shorter read/write lifetime of NAND flash memory. Because of this, were are not likely to see the hard drive go away anytime soon.

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