Information Technology Reference
In-Depth Information
Figure12.1: A partially-disassembled magnetic disk drive.
12.1
Magnetic disk
Magnetic disk is a nonvolatile storage technology that is widely used in laptops,
desktops, and servers. Disk drives work by magnetically storing data on a thin
metallic film bonded to a glass, ceramic, or aluminum disk that rotates rapidly.
Figure 12.1 shows a disk drive without its protective cover, and Figure 12.2
shows a schematic of a disk drive, identifying key components.
Each drive holds one or more platters, thin cylinders that hold the magnetic
Denition: platters
material.
Each platter has two surfaces, one on each side.
When the drive
Denition: surfaces
is powered up, the platters are constantly spinning on a spindle powered by a
Denition: spindle
motor. In 2011, disks commonly spin at 4200{15000 RPM (70{250 revolutions
per second.)
A disk head is the component that reads and writes data by sensing or
Denition: head
introducing a magnetic field on a surface. There is one head per surface, and as
a surface spins underneath a head, the head reads or writes a sequence of bits
along a circle centered on the disk's spindle. As a disk platters spins, it creates
a layer of rapidly spinning air, and the disk head floats on that layer, allowing
the head to get extremely close to the platter without contacting it. A head
crash occurs when the disk head breaks through this layer with enough force
Denition: head crash
to damage the magnetic surface below; head crashes can be caused by excessive
shock such as dropping a running drive.
In order to use the full surface, each head is attached to an arm, and all of
Denition: arm
a disk's arms are attached to a single arm assembly that includes a motor that
Denition: arm assembly