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(Enhanced Small Device Interface) A hard disk interface that transfers data in the one to three MByte/sec range. ESDI was the high-speed interface for small computers for a while, but has been superseded by IDE and SCSI drives. See hard disk.

hard disk

The primary computer storage medium, which is made of one or more aluminum or glass platters, coated with a ferromagnetic material. Although the terms "hard disk" and "hard drive" are used synonymously; technically, the disk spins inside the drive.

All computers have an internal hard disk for storage, and hard disks in external cases can be plugged into a USB, FireWire or eSATA port for additional storage. Slowly but surely however, hard disks are being replaced by non-mechanical drives (see solid state drive).

Today's hard disks are "fixed," which means their platters reside permanently in the drive. In the past, removable cartridges were used for backup and transport (see removable disk).

Storage... Not Memory
Hard disks are not the computer's main memory. Disks store programs and data until deliberately deleted by the user, but memory is a temporary workspace. To learn how this workspace is used to process data, see memory. For a summary of memory and storage types, see storage vs. memory.

Capacity and Speed
Hard disks rotate constantly from 4,000 to 15,000 RPM; however, to preserve battery or power, they can be configured by the user to turn off after a defined period of inactivity. Capacity is measured in bytes, and the largest drives have passed the terabyte threshold.

Speed is measured by how long it takes to begin transferring data; approximately three to 15 milliseconds (by comparison, CDs/DVDs take 80 to 120 ms) and the rate of transfer is measured in hundreds of megabytes per second. See hard drive capacity, access time and transfer rate.

Hard disks are pre-formatted at the factory, which divides the platters into identifiable sectors. For more details on disk structure, see magnetic disk, format program, hard disk defect management and drop protection.

Hard Disk Types
Over the years, several kinds of hard disks have been employed. Today, SATA drives are the most common, although SAS drives are also used. For more details, see SATA, SAS, SCSI and hard disk interfaces.

Non-Removable Internal Hard Disk
Hard disks use one or more metal or glass platters covered with a magnetic coating. In this drawing, the cover is removed.

First Hard Disk (5MB)
Part computer, part tabulator, in 1956, IBM's RAMAC was the first machine with a hard disk, which was extraordinary technology at that time. Each of its 50 platters two feet in diameter held a whopping 100,000 characters, the total equivalent to five megabytes today. (Images courtesy of IBM.)

First Personal Computer Hard Disk (5MB)
A fraction of the RAMAC's size, Seagate introduced the first hard disk with 5.25" platters in 1979. Today's platters are 3.5" for desktops and 2.5" for laptops. See ST506. (Image courtesy of Seagate Technology, Inc.)

Four Decades Later
Much less than entry level these days, but in 1998, this Seagate drive's 47GB was impressive. Four decades of development after the RAMAC let us store 100,000 times as much data on the same surface. All this is one platter today. (Image courtesy of Seagate Technology, Inc.)

World's Smallest Hard Drive
The size of a postage stamp, Toshiba introduced a 0.85" hard drive for mobile devices and shipped 2GB and 4GB units in 2005 and 8GB in 2007. However, solid state USB drives have long surpassed 8GB. (Image courtesy of Toshiba Corporation.)

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