Magnetic Recording

Data is written and read as a series of bits, the smallest unit of digital data. Bits are either a 0 or a 1, or on/off state if you prefer. These bits are represented on a platter’s surface by the longitudinal orientation of particles in the magnetically sensitive coating that are changed (written) or recognized (read) by the magnetic field of the read/write head. Data isn’t just shoveled onto a hard drive raw, it’s processed first, using a complex mathematical formula. The drive’s firmware adds extra bits to the data that allow the drive to detect and correct random errors.

Rapidly replacing longitudinal magnetic recording in new drive manufacture is a process called perpendicular magnetic recording. In this type of recording, the particles are arranged perpendicular to the platter’s surface. In this orientation they can be packed closer together for greater density, with more data per square inch. More bits per inch also means more data flowing under the read/write head for faster throughput.

Information is written to and read from both sides of the platters using mechanisms mounted on arms that are moved mechanically back and forth between the center of the platter and its outer rim. This movement is called seeking, and the speed at which it’s performed is the seek time. What the read/write heads are seeking is the proper track--one of the concentric circles of data on the drive. Tracks are divided up into logical units called sectors. Each sector has its own address (track number plus sector number), which is used to organize and locate data.

In the event a drive’s read/write head doesn’t arrive at the track it’s seeking, you may experience what’s called latency or rotational delay, which is most often stated as an average. This delay occurs before a sector spins underneath the read/write head, and after it reaches the proper track.

Source: http://www.pcworld.com/article/18693/how_it_works_hard_drives.html

Hard Drive Basics

A hard drive is a storage device that rapidly records and reads data represented by a collection of magnetized particles on spinning platters.

If a computer’s CPU is the brain of the PC, the hard drive is its long-term memory--preserving data programs and your operating system even while the machine is asleep or off. Most people will never see the inside of a hard drive, hermetically shrouded as it is in its aluminum housing; but you may have noticed an exposed PC (printed circuit) board on the bottom.

This PC board is where the brains of a drive are found, including the I/O controller and firmware, embedded software that tells the hardware what to do and communicates with your PC. You’ll also find the drive’s buffer here. The buffer is a holding tank of memory for data that’s waiting to be written or sent to your PC. As fast as a modern hard drive is, it’s slow compared to the data flow its interface is capable of handling.

If you took apart a desktop hard drive, you’d typically see from one to four platters, each of which would be 3.5 inches in diameter. The diameter of the platters used in hard drives for mobile products vary from as little as 1 inch for drives that are used in music players and pocket hard drives to the 1.8-inch and 2.5-inch platters typically used in notebook hard drives. These platters, also known as disks, are coated on both sides with magnetically sensitive material, and stacked millimeters apart on a spindle. Also inside the drive is a motor that rotates the spindle and platters. The disks in hard drives used in notebooks spin at 4200, 5400, or 7200 revolutions per minute; desktop drives being manufactured these days spin their disks at 7200 or 10,000 rpm. Generally speaking, the faster the spin rate, the faster data can be read.