A hard drive, also known as a hard disk drive or HDD, is a fundamental part of modern computers. The hard drive is where all of your programs and files are stored, so if the drive is damaged for some reason, you will lose everything on your computer.

A hard drive uses similar memory storage technology to cassette tapes and video tapes. You may already know that tapes store information on long, thin strips of tape with a delicate magnetic material on its surface. Likewise, hard drives contain round, mirror-like platters that are covered with that same magnetic recording medium.

The platters inside a hard disk drive are usually made of glass or aluminum. It is the polished magnetic material on the surface that makes the platter appear shiny, like a mirror. A clean, polished surface is critical to the proper functioning of the hard drive — even the smallest spec of dust can cause irreparable damage.

Just as a head inside of your cassette player or VHS player reads the data on the tape, a head inside your hard disk drive reads and writes data to the platters. This head is on an arm that is attached next to the platters, so that it can pivot back and forth over the them.

The average modern hard disk drive has several platters inside of it, stacked one on top of the other, like an Oreo cookie. There is a small gap between each platter, which allows each platter’s head to pass over it. The heads are all on the same arm, which has a separate branch for each head, rather like the tines of a fork turned on its side.

When you turn your computer on, the platters immediately begin to spin. The platters in a desktop computer hard disk drive typically get up to about 7,200 rotations per minute (rpm), while the hard drives in laptop computers usually run at 5,400 rpm. You may be able to hear the steady hum of your hard drive when the fan is not running.

When your computer is on but you are not retrieving or writing anything to the memory, the platters in the hard disk drive are always spinning. The arm with the heads on it, however, only begins to move when you run a program or open, save, or delete a file. This arm can move back and forth across the surface of the platter as many as 50 times in a single second, causing it to appear as a blur if you ever have the opportunity to watch. Because everything in the hard disk drive moves so quickly, the head never actually touches the platters, instead skimming just barely above them, supported by a cushion of moving air that is generated by the platters’ spinning.

The rapid motion of the platters and heads inside your hard disk drive make it susceptible to “head crash,” which is where the heads crash into the platters. Several different things can cause head crash. If dust gets into your hard drive and settles on the platters, it can actually cause the arm to bounce as the disk operates. The magnetic recording medium is extremely fragile, and is often ruined when the heads crash into the platters.

People who use laptops should also be very careful, as moving your laptop abruptly can cause head crash. Anytime you move your laptop when it is on, you should pick it up and set it down very gently. If you do not, eventually your hard disk drive may quit working altogether.