The primary computer storage medium until the 2000s, when solid state drives began replacing them in greater quantity. The first hard disk was introduced by IBM in 1956 (see RAMAC
A hard disk 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 platters spin inside the drive.
All computers used to have an internal hard disk for storage; however, today, storage can be solid state, especially in laptops (see SSD
). For more storage, external hard disks can be plugged into USB or an eSATA port.
Hard disks are "fixed disks," which means the platters reside permanently in the drive. In the past, removable cartridges for backup and transport contained the platters (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 (RAM) is a temporary workspace. To learn how this workspace is used to process data, see memory
and storage vs. memory
Capacity and Speed
As of 2022, a single hard drive holds up to 20TB (20 terabytes) of data. Speed is measured in megabytes per second for the transfer rate from drive to memory. It is also measured in latency, which is how long it takes to begin the transfer, typically from 3 to 15 milliseconds (ms). The platter has to rotate to the starting point of the transfer, and the read/write head has to move to that particular track. By comparison, CDs/DVDs take 80 to 120 ms.
Platters May Rotate Constantly
Disk platters rotate constantly at thousands of RPM unless they are configured to power down after a period of inactivity. This mechanical action is why hard drives are being replaced with solid state drives (see SSD
). See hard drive capacity
, access time
and transfer rate
Disks Come Pre-Formatted
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 emerged. Today, SATA drives are the most common, although SAS drives are also used. For more details, see SATA
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.
The RAMAC - First Hard Disk (1956)
Extraordinary technology for the 1950s, IBM's RAMAC was the first computer with a hard disk. Each of the 50 disk platters two feet in diameter held a whopping 100,000 characters. Today, the capacity of all 50 would fit on the head of a pin. (Image courtesy of IBM.)
First Personal Computer Hard Disk (1979)
Four Decades After RAMAC (1998)
In 1979, Seagate introduced the first hard disk for personal computers with 5MB on two 5.25" platters, the same capacity as all 50 platters in the RAMAC. Today, platters are 3.5" for desktops and 2.5" and 1.8" for laptops. See ST506
. (Image courtesy of Seagate Technology, Inc.)
World's Smallest (2005)
This 47GB Seagate drive held 100,000 times as much data as the RAMAC. Today, 20 times more storage is available on a flash drive (see USB drive
). (Image courtesy of Seagate Technology, Inc.)
No More Disks in the Future
The size of a postage stamp, the Microdrive was a less-than-one-inch hard disk for mobile devices with up to 8GB of storage (see Microdrive
This was cut out from a promotional mouse pad by an SSD vendor in the late 1990s. With SSDs gaining momentum at that time, hard disks were expected to disappear within a few years. Hardly the case as platter capacities have skyrocketed since then. Seagate has projected capacities from 50TB to more than 100TB within the next ten years.