Classic Computer Magazine Archive ANTIC VOL. 5, NO. 6 / OCTOBER 1986


"Like flying a Boeing 747 six inches above the ground"



Unlike floppy disks, a single hard disk can scan millions of bytes of information in seconds. While floppy disks and hard disks essentially do the same job-storing information-the hard disk goes about it in a completely different way.


A floppy disk is much like a thin, limp phonograph record covered with the same kind of material as recording tape. To read and write information to a floppy disk, a read/write (R/W) head is lowered onto the disk surface while it spins. Because the head physically touches the disk, friction becomes a factor in determining how fast we can spin the disk-and thus how fast we can access its information. Floppy disks typically spin at about 300 revolutions per minute (rpm).

A hard disk is another creature entirely. To start with, as the name implies, we have a hard disk on which to store information, as opposed to a floppy disk in a stiff paper cover. Typically the hard disk is an aluminum platter covered with a very thin coating of magnetic recording material. It also spins much faster, with some drives rotating as fast as 3,600 rpm. For a 5-1/4-inch disk, this is roughly 56 miles per hour. By comparison, a 12-inch phonograph record spinning at 33 1/3 rpm travels 1.9 miles per hour.

This is much too fast for the R/W head to touch the disk. Friction would soon burr up the entire drive mechanism. Therefore, the hard disk's R/W head cannot be allowed touch the disk at all. And here's how the head reads and writes to the disk without touching it.


Most hard disks function inside completely sealed cases that are filled with inert gas. A few hard disks "breathe" by drawing in air and filtering it, then exhausting the spent air.

When a physical object moves through a gas, some of that gas "sticks" to the object and moves along with it. As the hard disk spins, it sets up a very thin layer of gas that spins with it along the face of the disk. The R/W head in a hard disk drive is manufactured to take advantage of this thin layer of gas trapped right next to the spinning disk. The gas gets stuck in front of the R/W head and some of it is channeled underneath the head, forcing the head to "fly" above the surface of the hard disk.

Comparative sizes of Potential Contaminants

Information is stored on disks as magnetic patterns. And as you move away from a magnet, the strength of the magnetic field falls very quickly. There are electronic formulas that precisely predict this drop-off. So although the hard disk R/W head can't touch the disk itself, in order to store and retrieve information reliably we must place the head as close to the disk as possible.

The distance between the head and hard disk is so microscopic that virtually any object is larger. For example, no doubt you sometimes have needed to blow away a stray hair from the surface of a floppy disk. But if the distance between the hard disk and the R/W head was scaled up to one inch, the diameter of a typical human. hair would be over 16 feet!

Maybe while you removed the stray hair, you even blew cigarette smoke on the disk. At this scale, the smoke particles would range from a half inch to six inches across. If we allowed the cigarette smoke to actually touch the hard disk, some of it would become trapped between the head and the disk and cause scratches.

Just to get an idea of the precision and speed involved, picture a Boeing 747 flying at 600 miles per hour-six inches off the ground!

What would happen if the plane flew into the ground? No doubt it would be something like what happens during a "head crash" on a spinning hard disk. If for any reason the head touches the surface of the spinning disk while in operation, data will almost surely be destroyed from friction heat, which scrambles magnetism. In fact, the crash might be so violent that a physical scratch is gouged out of the disk surface. This renders the disk unusable and will cost mucho dollars to repair.

Therefore, any hard disk user should keep two things firmly in mind. First, treat your drive very gently and keep the head in lock position when you're not operating the disk. And second, make a floppy disk back-up of every hard disk file you wouldn't want to lose.