Number of Platters

Hard disks can have one platter, or more, depending on the design. Standard consumer hard disks, the type probably in your PC right now, usually have between one and five platters in them. Some high-end drives--usually used in servers--have as many as a dozen platters. Some very old drives had even more. In every drive, all the platters are physically connected together on a common central spindle, to form a single assembly that spins as one unit, driven by the spindle motor. The platters are kept apart using spacer rings that fit over the spindle. The entire assembly is secured from the top using a cap or cover and several screws. (See the spindle motor page for an illustration of these components.)

Each platter has two surfaces that are capable of holding data; each surface has a read/write head; both are used to store data.

From an engineering standpoint 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. More platters also means more mass, and therefore slower response to commands to start or stop the drive; this 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. See here for more on this trend.

The form factor of the hard disk also has a great influence on the number of platters in a drive. Even if hard disk engineers wanted to put lots of platters in a particular model, the standard PC "slimline" hard disk form factor is limited to 1 inch in height, which limits the number of platters that can be put in a single unit. Notebook 2.5" hard drives are even more limited; a 9.5mm height is most common though there are several high-capacity 12.5mm drives floating around. Of course, engineers are constantly working to reduce the amount of clearance required between platters, so they can increase the number of platters in drives of a given height.

Next: Platter Substrate Materials