Disk Platters and Media
Every hard disk uses one or more (generally more than one) round, flat disks called platters, coated on both sides with a special media material designed to store information in the form of magnetic patterns. Each surface of each platter on the disk can hold billions of bits of data.
Platters are composed of two main substances, a substrate material that forms the bulk of the platter and gives it structure and rigidity, and a magnetic media coating which actually holds the magnetic impulses that represent the data.
The quality of the platters and particularly, their media coating is critical. The size of the platters in the hard disk is the primary determinant of its overall physical dimensions, also generally called the drive's form factor; most drives are produced in one of the various standard hard disk form factors.
Sometimes hard disks are referred to by a size specification. If someone is having a 3.5-inch hard disk it means it usually refers to the disk's form factor, and normally, the form factor is named based on the platter size. The earlier hard disks had a nominal size of 5.25" but now days the most common hard disk platter size is 3.5".
Laptop drives are usually smaller, due to the expected small size and less weight of it. The platters on these drives are usually 2.5" in diameter or less; 2.5" is the standard form factor, but drives with 1.8" and even 1.0" platters are becoming more common in mobile equipment.
Though drives extend the platters to as much of the width of the physical drive package as possible, to maximize the amount of storage they can pack into the drive yet the trend overall is towards smaller platters. There are the main reasons why companies are going to smaller platters even for desktop units:
- The rigid and stiff platters are more resistant to shock and vibration, and are better-suited for being mated with higher-speed spindles and other high-performance hardware. Reducing the hard disk platter's diameter by a factor of two approximately quadruples its rigidity.
- Reduced size of the platters reduces the distance that the head actuator must move the heads side-to-side to perform random seeks. This improves seek time and makes random reads and writes faster.
- The latest hard disk spindles are increasing in speed performance reasons. Smaller platters are easier to spin and require less-powerful motors as well as faster to spin up to speed from a stopped position.
The smallest hard disk platter size available today is 1" in diameter. IBM's amazing Micro drive has a single platter and is designed to fit into digital cameras, personal organizers, and other small equipment. The tiny size of the platters enables the Micro drive to run off battery power, spin down and back up again in less than a second.
From an engineering point of view more platters also means more mass and therefore slower response to commands to start or stop the drive. It can be compensated for with a stronger spindle motor, but that leads to other tradeoffs.
In fact, the trend recently has been towards drives with fewer head arms and platters, not more. Areal density continues to increase, allowing the creation of large drives without using a lot of platters. This enables manufacturers to reduce platter count to improve seek time without creating drives too small for the marketplace.
The form factor of the hard disk also has a great influence on the number of platters in a drive. There are several factors that are related to the number of platters used in the disk. Drives with many platters are more difficult to engineer due to the increased mass of the spindle unit, the need to perfectly align all the drives, and the greater difficulty in keeping noise and vibration under control.