The primary computer storage medium, which is made of one or more aluminum or glass platters, coated with a ferromagnetic material. Most hard disks are "fixed disks," which have platters that reside permanently in the drive. Almost all computers have an internal hard disk, and external units can be plugged in for additional storage or backup.|
The other type of hard disk is a "removable disk" encased in a cartridge, allowing data to be ejected from the drive for external storage or transfer to another party. Before high-speed Internet connections were common, removable SyQuest, Jaz and Zip cartridges were routinely shipped via the post office (see removable disk).
Three Major Categories: PATA, SATA and SCSI
Most hard disks are Parallel ATA (PATA), Serial ATA (SATA) or SCSI. SCSI drives have traditionally been found on servers and high-performance workstations and were the first drives used in fault-tolerant RAID systems. Today, ATA drives are widely used for RAID arrays. See IDE, PATA, SATA, SCSI and RAID.
Hard drives are low-level formatted at the factory, which records the original sector identification on the platters (see format program). See hard disk defect management.
Hard disks provide fast retrieval, because they rotate constantly at high speed from 4,000 to 15,000 RPM. Either to preserve battery life in laptops or to promote longevity, hard disks can be configured to turn off after a defined period of inactivity.
It Started in the Mid-1950s
In 1956, IBM introduced the RAMAC hard disk with platters two feet in diameter that held the equivalent of 100,000 bytes. In the 1980s, desktop computer hard disks were introduced with 5MB using 5.25" platters (see ST506). Today's entry-level drives have at least 8,000 times more capacity. Platter size was reduced to 3.5" for desktops, 2.5" for laptops and 1" for handhelds. In 2004, Toshiba introduced the 0.85" drive (see below). See magnetic disk, floppy disk, Microdrive, drop protection and CAV.
TYPES OF HARD DISKS
Type of Encoding Rate Maximum
Interface Method** (Per sec) Capacity
SATA (IDE) RLL 150-300MB 2TB
PATA (IDE) RLL 3-133MB 1TB
SCSI RLL 5-320MB 300GB
SAS RLL 375-750MB 2TB
Older Interfaces Range
IPI RLL 10-25MB 200MB-3GB
ESDI RLL 1-3MB 80MB-2GB
SMD RLL 1-4MB 200MB-2GB
IDE RLL 1-8MB 40MB-1GB
ST506 RLL RLL 937KB 30MB-200MB
ST506 MFM 625KB 5MB-100MB
** Most disks use RLL, but encoding methods are
not prescribed by all interfaces.
Hard Disk Measurements
Capacity is measured in bytes, and speed is measured by transfer rate in bytes per second (see above) and access time in milliseconds (ms). Hard disk access times range from 3 ms to about 15 ms, whereas CDs and DVDs range from 80 ms to 120 ms.
Hard disks use one or more metal or glass platters covered with a magnetic coating. Although there has been a variety of removable hard disks over the years, a computer's primary hard disks are fixed inside the drive. The entire unit is removed only to be replaced or repaired. In this drawing, the cover is removed.
Part computer, part tabulator, in 1956, IBM's RAMAC was the first machine with a hard disk, which was extraordinary technology of the times. Each of its 24" diameter platters held a whopping 100,000 characters (they were not bytes then) for a total of five million characters. (Images courtesy of International Business Machines Corporation. Unauthorized use not permitted.)
Seagate introduced the first hard disk for personal computers in 1979. At 5MB, the ST506 held 10 times as much as the RAMAC at a fraction of its size. (Image courtesy of Seagate Technology, Inc.)
Entry level these days, but in 1998, this Seagate drive's 47GB was impressive. Four decades of research and development let us store 100,000 times as much on the same platter surface. Even more impressive is that this much data are stored on one side of only one platter today. (Image courtesy of Seagate Technology, Inc.)
Toshiba introduced the first 0.85" hard drive and shipped 2GB and 4GB units in 2005. About the size of a postage stamp, it was named the world's smallest hard disk in Guinness Book of Records. Targeting use in digital cameras, mobile phones, camcorders and MP3 players, capacity was increased to 8GB in 2007. (Image courtesy of Toshiba Corporation.)