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How hard disks work
Internal of a hard disk drive
If you are to dismantle the hard disk drive by opening the top casing (after removing all the necessary screws), the first thing you’ll see is a spindle holding one or a number of mirror-like hard rotating platters (commonly called data platter). The platters could be made to spin at an extremely high speed, technically between 5,400 to 10,000 revolutions per minute (RPM). An extremely thin magnetic coating is layered onto the surface of the platter that is polished to mirror-type smoothness.
The platter is usually made of glass or ceramic (modern platter may use titanium). Unlike a floppy disk, the platter cannot be bent or flipped and hence we coin it as “hard disk” or “hard drive”. Commonly a hard disk contains 1 to 10 identical platters that are stacked in parallel to form a cylinder. There is usually one Read Write (RW) head designated per platter face, and each head is attached to a single actuator shaft which moves all heads in unison and performs a uniform synchronous motion during reading or writing of data.
Data on a hard disk is stored in microscopic areas called magnetic domains on the magnetic material. Each domain stores either a 1 or 0 value. Similar to a floppy disk, a hard disk records its data in concentric circles or tracks, which are numbered from the outermost edge to the innermost edge of the platter. These tracks are further subdivided into smaller units called sectors which typically store 512 bytes of data each. Zoning may be needed to further optimise the data storage as the outer circumference would normally pack more sector units than the inner circumference.
Read Write Head
The RW head is the key component that performs the reading and writing functions. It is placed on a slider which is in term connected to an actuator arm which allow the RW head to access various parts of the platter during data IO functions by sliding across the spinning platter.
The sliding motion is derived by passing a current through the coil which is part of the actuator-assembly. As the coil is placed between two magnets, the forward or backward sliding motion is hence derived by simple current reversal. This location of the platter (just like the landmark along the road) is identified and made possible by the embedded servo code written on the platter.
To write a piece of information to the disk, an electromagnetic flux is transmitted through the head which hovers very closely to the platter. The RW head suspends on a thin cushion of air which the spinning platter induces. This designed distance between the head and platter is called the flying height. It can literally measure to a few millionths of an inch. A good analogy is to try to imagine flying a Boeing 747 with about 6 inches above ground level.
It is therefore easy to understand that if the RW head happens to “knock” on the spinning platter out of design specification, a Read Write head crash occurs.
Read Write Function of Disk
As the head writes data onto the disk, it changes its magnetic polarization to induce either a one or zero value. During a read request, data is interpreted when the magnetic fields on the platter brings about an electrical change (as a result of change in electrical resistance of some special material property) in the read-head that passes over it. These electrical fields are then encoded and transmitted to the CPU to be processed and read by the system.
Parking of RW Head
The read/write head will move to the parking zone when the computer is switched off.
When the computer is switched off, the head is usually pulled to a safe parking zone to prevent the head from scratching against the data zone on platter when the air bearing subsides. This process is called a parking and different techniques have been implemented in various hard disks to handle the take offs and landings. In a Ramp load/unload design, a lifting mechanism parks the head outside.