The Physics
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Angular Speed of a Hard Disk Drive

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Meyers, Mike. All In One CompTIA A+ Certification Exam Guide.Sixth ed. Ed. Timothy Green. New York: Osbourne, McGraw-Hill 2007. "It has to be, as the read/write heads actually float on a cushion of air above the platters, which spin at speeds between 3500 and 10,000 r.p.m." 3500–10,000 rpm
Hard Disk Definition, TechEncyclopedia, TechWeb, 2007. "Hard disks provide fast retrieval because they rotate constantly at high speed from 5000 to 15,000 rpm." 5000–15,000 rpm
Schultz, Brian E. Innovative Power Solutions in Hitachi Disk Drives [pdf]. Hitachi Global Storage Technologies, 2006. "Test data shows that when coupled with Enhanced ABLE, HiVert enables Hitachi Travelstar 2.5- and 1.8-inch hard disk drives which operate at very high performance levels to consume comparable or even lower amounts of power than 4200, 5400 and more recently 7200 rpm offerings from other suppliers." 4200–7200 rpm
Chalmers, Brian D.; Mackenzi, Colin A.; Kapor, Kishore K. Rotary Voice coil Micro-Hard Disk Drive System. US Patent 4890174. UK: Rodime PLC. Date Filed: Mar. 18, 1987. Date Issued: Dec. 26, 1989. "A three-phase brushless DC motor rotating at 3600 r.p.m. is used for driving the disks at constant speed." 3600 rpm

A Hard Disk Drive (HDD) is a device invented by IBM scientists in San Jose, California 1956. The late Reynold Johnson, a longtime IBM employee, is commonly referred to as the father of the hard disk drive. The device stores data by manipulating and reading the change in magnetic polarization generated from the surface of a rapidly revolving disk. These disks, or platters are usually made of glass, aluminum, or any cobalt-based alloy, smoothly coated with a magnetic medium. Magnetic fields from the medium are read/written by mechanisms, called heads, floating above the rotating platter. The distance between the heads and the disk surface is less than the thickness of a fingerprint, thus making the data easier to read and more densely packed onto the drive.

HDDs are currently organized into two major categories of interfaces--Advanced Technological Attachment (ATA) and Small Computer System Interface (SCSI or "Skuzzy"). ATAs are the most common type of hard drives while SCSI are usually used in servers, and are known commercially as "enterprise HDDs." Remaining HDDs are generally organized into an unofficial third category called Redundant Array of Inexpensive Drives (RAID) because they divide and replicate data among multiple drives.

HDDs are extremely delicate. The platters should never be exposed to outside air since even the smallest dust particle could disrupt the magnetic field readings. Therefore, all hard drives use a filtered opening to keep interior and exterior air equalized. In addition, it is advised that HDDs temperature never go above 38 °C; once the temperature reaches 43°C, the drive would experience overheating, instability, and reduction of component life. This is why many HDDs are mounted into computer processors with fans.

Hard drives have become increasingly efficient and are storing more data than ever before. The characteristics that have the greatest impact in the HDD's storage capacity include its platter speed, density and composition. As the speed of a rotating platter increases, as it have been recently, the hard drive stores data at faster rates, and its physical density increases, the drive's storage capacity increases as well. This is why HDD platters were previously composed of iron (III) oxide, which is less dense, as opposed to today's cobalt alloy, which is denser. Furthermore, the earliest HDDs spun less than 1000 revolutions per minute (rpm). Most of the HDDs in the 1980s were powered by a spindle DC motor that spun at a constant speed of 3600 rpm or less. Today's hard disk drives are applied to many areas of technology ranging from company servers (enterprise HDDs) to home computers (desktop HDDs), and from digital video recorders, digital audio players and video game consoles (consumer HDDs) to laptops and cell phones (mobile HDDs). Therefore, their angular speeds today depend much on the market/usage at which they are targeted. Consumer and especially mobile hard drives, for instance, are physically smaller and most revolve at either 4200, 5400, or even 7200 rpm. Most desktop HDDs revolve at 5400 rpm or 7200 rpm, while few revolve at 10,000 rpm. However, these 10,000- and 7200-rpm drives tend to be more expensive, more noisy, and give off more heat. Enterprise HDDs, since they are usually used by corporations, tend to regularly revolve at 7200 to 10,000 rpm while the fastest can spin up to 15,000 rpm. Today hard disk drives can save 100-200 gigabytes worth of data per platter, and many engineers expect the HDD to spin at 20,000 rpm in the near future.

Furthermore, several other HDD characteristics have also been used recently to manipulate the capacity and the speed at which the device stores data. Due to market pressures, companies such as Hitachi Global Storage Technologies have created multi-platter HDDs (consisting five disks) that can store up to 1000 GB or 1 Terabyte. An alternative method of magnetic reading is also currently being experimented, and promises even greater memory density per platter. The method is referred to as Perpendicular Magnetic Reading (PMR) because data is read perpendicularly to the disk rather than longitudinally. PMR aligns the poles of the magnet, thus increasing the magnetic elements stored in one area. However, the method is extremely susceptible to the high thermal temperatures and energy caused by superparamagnetism.

Simon Chen -- 2007