A Basic Understanding of Platters
Hard disks can have one or more platters depending on their design. Normal consumer hard drives, usually have between one and five platters. Some high-end drives, typically found in servers, can have as many as a dozen platters. In every hard drive, all the platters are actually linked together on a common central spindle, thus forming one single assembly that spins as one unit that is driven by the spindle motor. The platters are kept apart using spacer rings that fit over the spindle. The complete assembly is held from the top using a cap or cover and some screws. See the picture below of the interior of a hard drive.
Each platter has two surfaces that are able to hold data; each surface has a read/write head. Typically both surfaces of each platter are used, however that is not always true. Newer drives sometimes leave a surface vacant in order to create a drive of a particular capacity in a family of drives.
The amount of data that can be stored on a given amount of hard disk platter space continues to increase, allowing the formation of large capacity drives without using lots of platters enabling manufacturers to decrease platter quantity and improve seek time without creating drives too small for the marketplace.