hard disk drive

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hard disk drive

(storage)
(HDD) A disk drive used to read and write hard disks.
This article is provided by FOLDOC - Free Online Dictionary of Computing (foldoc.org)

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 used to have an internal hard disk for storage; however, today, storage can be solid state (see SSD). External hard disks can be plugged into USB or a eSATA port for more storage.

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. For a summary of memory and storage types, see storage vs. memory.

Capacity and Speed
Capacity is measured in bytes, and the largest drives hold up to 10 terabytes. Speed is measured by transfer rate in megabytes per second as well as latency: how long it takes to begin transferring data, typically 3 to 15 milliseconds (ms). By comparison, CDs/DVDs take 80 to 120 ms.

The platters rotate constantly at thousands of RPM; however, to preserve battery, laptops default to powering them down after a set period of no activity. This mechanical action is why hard disks are slowly fading into history and being replaced with SSDs (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
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 Computer (1956)
Extraordinary technology, IBM's RAMAC was part computer, part tabulator and the first computer with a hard disk. (Image courtesy of IBM.)







Five Megabytes Total
Each of the RAMAC's 50 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)
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.)







Four Decades After RAMAC (1998)
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.)







World's Smallest (2005)
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).
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References in periodicals archive ?
Removing left aileron well inspection panel revealed the roll/aileron trim motor actuator arm bungee totally sheared off of motor drive shaft end; the bungee assembly was still attached.
In experiments, during the changes in position, there was monitored the influence of a static load placed on the arm of the experimental assembly on the dynamics of the actuator arm stabilization process utilizing the power based on the pneumatic artificial muscles in antagonistic involvement.
Attaching a sensor to it and measuring the length of stroke of the actuator arm could be used to measure process parameter drift and raise an alert when it reaches an alarm limit.
The latter problem I have fixed as it was only the safety actuator arm being loose and fouling the trigger mechanism.
Valvestock, the specialist valve and actuator arm of Pipe Center, has established an industrial pumps offering to target the specialist requirements of large-scale projects across the UK, support OEMs and deliver a unique multi-product offering to industrial end users.
* If the cocking actuator lever doesn't fully engage or the cocking actuator arm slot isn't parallel to the mount so that the cocking bracket can seat properly, repeat the instructions in WP 0016.
Pneumatic artificial muscles (PAMs) act against themselves and resultant position [phi] of the actuator arm is given by equilibrium of their tensile forces (FI, [F.sub.2]) according to different pressures ([p.sub.1], [p.sub.2]) in muscles (Fig.
The Spoon is attached to the Spoon Arm, which has a travel slot milled into it that allows the Spoon and its actuator arm to travel along the slot as the mold pulls the Spoon away from the stationary half.
A full-size keyboard, totally enclosed actuator arm and battery maintenance LED indicators add functionality.