Term of the Moment
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Definition: hard disk
The primary computer storage medium until the turn of the century when solid state drives began replacing them. Hard disks emerged in corporate datacenters in the late 1960s and began to flourish in the 1970s alongside tape drives, which were the primary storage medium (see magnetic tape).
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 storage vs. memory.
Capacity and Speed
As of 2024, the largest hard disks hold 26 to 30 terabytes of data. A drive's speed is measured in gigabits per second for the transfer rate from drive to memory. Latency is also a factor, which is the time it takes to begin the transfer, typically 3 to 15 ms (milliseconds). The read/write head must move to the required track and wait until the platter rotates to the start of the sector. By comparison, CDs and DVDs have 80 to 120 ms of latency. See sector.
Platters 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, SAS, SCSI and hard disk interfaces.
Non-Removable Internal Hard Disk
The RAMAC - First Hard Disk (1956)
First Personal Computer Hard Disk (1979)
Four Decades After RAMAC (1998)
World's Smallest (2005)
No More Disks in the Future
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 storage vs. memory.
Capacity and Speed
As of 2024, the largest hard disks hold 26 to 30 terabytes of data. A drive's speed is measured in gigabits per second for the transfer rate from drive to memory. Latency is also a factor, which is the time it takes to begin the transfer, typically 3 to 15 ms (milliseconds). The read/write head must move to the required track and wait until the platter rotates to the start of the sector. By comparison, CDs and DVDs have 80 to 120 ms of latency. See sector.
Platters 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, SAS, SCSI and hard disk interfaces.
Hard disks use one or more metal or glass platters covered with a magnetic coating. In this drawing, the cover is removed.
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.)
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.)
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.)
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, and Seagate has projected disks from 50TB to 100TB within the next ten years.