How Hard Drives Work
Like videotape and floppy disks, hard disks are magnetic media. Magnetic impulses with either positive or negative polarities represent the data stored on the magnetic layer of a hard disk's round, rigid platters. A positive polarity could represent 1 in the language of binary code and a negative polarity could represent a 0. Each of these 1s and 0s is known as a bit.
The hard-disk platters -- where the bits are stored -- are made of glass or aluminum. A typical hard drive contains several of these 3.5-inch platters, which can contain tens of billions of individual bits. The higher the areal density of the hard disk's platters, the more bits that can be packed into each square inch of platter real estate.
A platter is segregated into tens of thousands of concentric tracks. Because so much information can be stored in one track, the tracks are broken down into smaller units called sectors. Each sector can hold about 512 bytes of data, or 4096 bits (there are eight bits to a byte.)
Grabbing Info From a Hard Drive
Let's say you want to retrieve a Word document from your hard drive. Hopefully, the information is stored in the hard drive's limited disk cache (512KB to 4MB in size), where it can be retrieved at lightning speed. If it's a document you haven't used in a while, it probably won't be in the cache and will be retrieved from one or more of the disk platters.
Word, your OS, and the BIOS collaborate to tell the hard drive's controller and logic board where the information is stored. (Note: The controller and logic board are the circuitry you see on the bottom of your hard drive's casing.)
Let's get mechanical
Disk platters are mounted in a stacked formation on a spindle (sort of like an axle.) A spindle motor turns the platters at breakneck speed, typically between 5,400 and 7,200 times per minute, but as fast as 15,000 times per minute.
The platters spin so that the appropriate sector or sectors containing the data that constitutes our Word document can be positioned underneath one of the drive's reading heads. There's one head per platter, and all the heads move in unison.
Each head is mounted onto a slider, which is mounted onto an arm. A mechanical device called an actuator controls each hard drive arm. The actuator moves the arm to the correct position on the spinning platter, which puts the head in the correct position. If you've ever taken a hard drive apart and watched this process, you've probably been amazed by the fact that the arm can move the head from the edge to the center of a platter some 50 times per second.
The reading head (these days reading and writing heads are separate) flies about 2/1,000,000 of an inch above the disk surface. As it passes over the appropriate disk sectors, it interprets the magnetic pulses and converts them to electrical pulses that can be interpreted as 1s and 0s.
Hard-drive noise demystified
Our Word document is, in all likelihood, scattered across different sectors, tracks, and even platters. When writing to a hard disk, data is stored wherever there's room, not in a line. This is why your hard drive makes so much noise as it searches for data -- the heads are flying all over the place. When you defrag your hard disk, you're trying to place individual files into tidier order.