The hard drive, also called a hard disk, is your computer's main storage device. Think of it like a file cabinet. Windows is installed on your hard drive. Your documents, pictures, music, financial records, etc. are all stored on your hard drive. A computer can have more than one hard drive but the primary hard drive is known as the C drive in Windows. Inside a hard drive is a rapidly spinning disk (or disks) called platters that are coated with a magnetic material. The magnetic surface is manipulated by a tiny read/write head that sweeps across the platters. Because it is stored magnetically, a hard drive does not require any power to maintain the data on the platters once it is written. You can write something to a hard drive, unplug it and ship it across the country, and the data will be readable by the recipient.

Hard drives are quite fragile while in use. Any sudden physical shock, like from moving or dropping your computer while its running, can cause the heads to slam into the platters, much like moving a record player while its playing will damage the record and needle. This is known as a head crash. If this happens, the data stored where the head contacted the platter can be lost or the entire drive can become unusable. Because hard drives are fragile and they contain all your precious data, its very important to back up your hard drive regularly.

The platters inside hard drives used in desktop computers measure about 3.5" across while the drives used in laptops are about 2.5" across. Due to their larger physical size, desktop hard drives hold more data than laptop hard drives but the capacity of both is always increasing. At any point in time, the largest desktop hard drive you can purchase is usually between two and three times as large as the largest laptop hard drive available.

Below is a typical desktop and laptop hard drive. Both have an IDE connector for connecting the drive to the computer but desktop and laptop IDE are a little different. Desktop IDE uses a 40 pin connector for data (connected to the motherboard) and a separate 4 pin connector for electricity from the power supply. Laptop IDE uses a single set of 44 smaller pins instead that connect directly to a socket inside the laptop without using any cables.

Below are two laptop hard drives. The one on the left is IDE with its 44 pin power+data connection. The drive on the right is a newer type of connection called SATA. Unlike IDE, SATA connections are identical on desktop and laptop drives. SATA uses a 7 pin data cable (connects to motherboard) and a 15 pin electrical cable from the computer's power supply.

Below are a 3.5" desktop hard drive and  2.5" laptop hard drive with their covers removed. The platters spin constantly as long as the drive is turned on, whether the computer is accessing it or not. The ticking or grinding sound you hear when your computer is busy is caused by the head assembly pivoting rapidly left and right across the platters as it repositions itself to read and write different parts of the platters. The drive on the left has three platters (six sides total) so it has six read/write heads on the head assembly to be able to access the top and bottom surface of all platters. The drive on the right only has two platters so it has four read/write heads. The number of platters has a part in determining the storage capacity of a drive. For example, a manufacturer may develop a platter capable of storing 250 GB of data. They will then build a line of drives holding 250 GB, 500 GB, or 750 GB of data containing 1, 2, or 3 platters respectively.

All of the hard drives shown so far are designed to be installed inside a computer. External hard drives are designed to sit outside of the computer and connect to it by a cable, most often a USB cable. Regardless of whether it goes inside or outside of a computer, its still an internal hard drive at heart. Pictured below is a 2.5" laptop hard drive and a USB hard drive based on a 2.5" drive. The only difference is the protective case and a small board to allow the IDE or SATA drive inside communicate over USB. Since 2.5" drives require very little power, the USB drive below only needs to be plugged into the computer's USB port to operate. External drives based on the larger 3.5" desktop drives are physically larger and hold more data, but require electricity from your wall outlet which means more cords and yet another power transformer.

The picture below shows an IDE hard drive connected to the motherboard and power supply. An IDE CD-ROM drive (pictured at the bottom of this page) would connect in the same exact manner. This motherboard has two IDE ports. Two drives (any combination of hard drive and optical drive) can be connected to each port. Therefore this board can support four IDE drives. 

Below is a short video of two hard drives running with their top covers open. The video starts with the first drive already spinning full speed and that is the sound you hear. The rapid back and forth movement of the read/write heads is called seeking. This drive's seeking is very quiet but some drives make a soft ticking or scratching sound when they seek. The second video shows a different drive spinning its platters up to speed from a dead stop. Its heads then bang back and forth wildly before giving up and stopping the platters.