How It Works: Hard Drives

It's one of your PC's vital organs. Here's what keeps the heart of the computer beating.

Robert L. Hummel

Hard Drive: a storage device that rapidly records data as magnetic pulses on spinning metal platters.

If a computer's CPU is the brain of the PC, the hard drive serves as the heart, pumping vital data to the rest of the system. As the workhorse component of virtually every computer, the hard drive is also the most mysterious. Most people never see the inside of a hard drive, shrouded in its aluminum housing, though they might be intimately familiar with the files and programs it stores, copies, moves, and deletes for them.

• Hard drives provide long-term storage for data on your PC.
• Storage capacities for new drives grow every year (the largest has reached 80GB this year), but the physical size of drives remains relatively constant.
• The faster a drive spins, the faster you can access and transfer data.
• As ever-larger hard drives reach the market, the cost of hard drives (measured as dollars per megabyte of storage) drops.

Hard drives provide the data storage on which all modern computers depend. A hard drive stores information by applying a magnetic field to the moving surface of a disk coated with a magnetic material.

The underlying principle of hard drives--the use of magnetism to store information--is very similar to that used in a tape or video recorder. A hard drive stores digital data as magnetized spots on the surface of the disk. A bit (your data is composed of bits) represents a 0 when magnetized in one orientation and a 1 when magnetized in the opposite orientation.

Each individual hard disk inside a drive is called a platter. A large-capacity hard drive will typically contain several 3.5-inch platters and will use both sides of each platter for storage. The drive contains a motor that spins the platters at speeds from 4500 to 15,000 rotations per minute.

Hard drives use a recording device called a head to write data to and read data from each platter surface. The drive positions a head, on a movable arm, a microscopic distance above the surface of each side of each platter, so a drive with five platters would have ten separate heads on ten arms.

Other elements in the head assembly read the recorded data by sensing the faint magnetic field from each magnetized bit speck as it passes under the read element.

The drive records data in concentric circles, called tracks,and divides each track into segments known as sectors. You can think of a track as a bookshelf with each segment on the track representing an individual book. If the operating system needs a file located at a particular track and sector, it sends a request to the hard drive to retrieve the data at that particular address.

How Hard Drives Read, Write, and Do Arithmetic

When an operating system sends data to the hard drive to be recorded, the drive first processes the data using a complex mathematical formula that adds extra bits to the data. Those bits aren't wasted space: Later, when the data is retrieved, the extra bits allow the drive to detect and correct random errors caused by variations in the drive's magnetic fields.

Next, the drive moves the heads over the appropriate track on a platter. The time it takes to move the heads is called the seek time.Once over the correct track, the drive waits while the platters rotate the desired sector under the head. The amount of time that takes is called the drive's latency. The shorter the seek time and latency, the faster the drive can do its work.

When the drive electronics determine that a head is over the correct sector to write the data, the drive sends electrical pulses to that head. The pulses produce a magnetic field that alters the magnetic surface of the platter. The variations recorded there now represent the data.

Reading data complements the recording process. The drive positions the read portion of the head over the correct track, and then waits for the correct sector to orbit around. When the particular magnetic specks that represent your data in the right sector and track pass under the read head, the drive's electronics detect the small magnetic changes and convert them back into bits. Once the drive checks the bits for errors and fixes any it sees, it sends the data back to the operating system.

What's in an Interface?

The hard drive interface is simply the hardware that manages the exchange of data between your computer and the hard drive. You're likely to encounter only one type of interface with most PCs: the Advanced Technology Attachment, also known as the ATA (or IDE) interface. Hard drives that use this interface come in a variety of flavors, named Ultra ATA, Ultra DMA, or EIDE, depending on which vendor you look at. The distant second-place interface belongs to SCSI, used in most servers and in older Apple Macintosh computers.

The original ATA interface supported a maximum transfer rate of 8.3MB per second. ATA-2 boosted the maximum throughput to 16.6MB per second. Though not an official standard, Ultra DMA-33 and Ultra DMA-66 are generally accepted by the hard drive industry to define interfaces with a maximum transfer speed of 33MB and 66MB per second, respectively. Recently, Seagate announced that it had begun shipping its Barracuda ATA III drive with the new Ultra ATA-100 interface targeted primarily at the traditionally SCSI domain of RAID servers.

Cheap Hard Drives Push the Envelope

The original IBM PC-XT came with one of the first mass-produced hard drives: Massive 5.25-inch platters provided a whopping 10MB of storage--a huge amount for its day. Nearly 20 years later, the latest hard drive with 3.5-inch platters can store upwards of 70GB. Smaller, 2.5-inch notebook computer drives can store in excess of 25GB. And tiny 1-inch-wide Microdrives designed for digital cameras, MP3 players, and handheld computers can hold up to 1GB. Accompanying the transition to smaller size and greater capacity has been a dramatic decrease in the cost per megabyte of storage, making even the largest hard drives affordable to average computer users.

You can buy hard drives specially designed for three distinct uses: desktop PCs, notebook computers, and servers. The distinguishing attribute is not drive capacity--more is always better--but the other factors that determine performance: the drive's motor speed, the on-board memory, the drive's interface, and power consumption.

Specialized Drives for Each Task

Value is the major concern in the consumer desktop market. A computer system built for the bottom line will probably include a 5400-rpm drive with an IDE interface, representing the sweet spot on the price-performance curve today, averaging under $300 for a good amount of storage. Higher-performance workstations usually ship with 7200-rpm drives, with 10,000-rpm drives expected to be common within a year.

In the mobile market, the overriding concern is power conservation. Slower-rotation drives can minimize the hard drive's power drain on the laptop battery. The maximum storage afforded by notebook drives tends to lag behind that of desktops, due to smaller platter size.

In the server market, the primary goal is performance. In a commercial server that handles online sales, for example, the drive must be able to handle multiple requests for data from different sectors as quickly as possible. Here, you'll commonly find SCSI drives (usually set up in a RAID array) spinning at 10,000 rpm, with 15,000-rpm drives just beginning to emerge. These drives are only limited by their inability to dissipate the amount of heat they produce and to keep the noise they generate at a tolerable level.

Prices Always Going Down

In the previous three years, drive prices dropped through the floor even as drive capacity soared. Vendors warn that they have already removed most of the excess cost from drives by integrating the electronics, reducing component counts, and increasing reliability. With 150 million drives expected to ship this year and predictions for 200 million to ship annually within two years, the mass market for drives seems to never stop growing.

Sub-$1000 desktop systems often boast 8GB or larger drives, about the minimum you should specify for any system. For midrange desktops in the $1500 neighborhood, 40GB drives are more typical. But one vendor predicts that, over the next five years, the proportion of the price of a computer system allocated to the hard drive will actually rise a bit. The price of other components will drop while the cost of the hard drive itself remains relatively constant. It's not all bad news: As capacity continues to rise, the cost per megabyte will continue to drop.

If you're happy with your current desktop system, but simply want to boost your disk storage capacity, you can add a new drive for a song. Unless you're buying drives to drop into your company's big server, any popular drive will give you good performance (5400- to 7200-rpm spindle speed) and good capacity (20-40GB) for the money. At press time, the street price for the DiamondMax 30GB drive, for example, is under $200, while about $260 will get you 40GB from Maxtor. IBM sells a 75GB drive, targeted at enterprise servers, for about $500.

When you shop for a new drive, you'll see that vendors and stores often tout the "maximum transfer rate" or "burst speed" of drives. Those numbers don't tell you much; they just describe the specifications of the interface. The numbers are often used as marketing propaganda; most of the time, your drive won't reach that level of data transfer speed.

The Future of Hard Drives

Prior to the commercial development of the giant magnetoresistive (GMR) heads that power today's largest-capacity drives, hard drive capacity doubled about every 18 months. Over the next five years, vendors predict, GMR technology will double hard drive capacity every 12 months.

Further out, vendors will have to defy the laws of physics and learn to make smoother platters and smaller heads. One tough challenge will involve overcoming the so-called super-paramagnetic limit, at which the bit size on the drive becomes so small that random thermodynamic fluctuations can cause data loss. And some storage companies are moving even further afield in the search for new kinds of storage. Nonetheless, according to hard drive vendors, the future of the industry is bright--especially for the consumer.

Robert L. Hummel is a New Hampshire-based computer programmer, author, and commentator.