Introduced by IBM in 1956, HDDs became the dominant secondary storage device for general-purpose computers by the early 1960s. The revenues for SSDs, most of which use NAND, slightly exceed those for HDDs. Though SSDs have nearly 10 times higher cost per bit, they are replacing HDDs in applications where speed, power consumption, small binary option indicator v2020, and durability are important.
The primary characteristics of an HDD are its capacity and performance. The two most common form factors for modern HDDs are 3. 5-inch, for desktop computers, and 2. The first production IBM hard disk drive, the 350 disk storage, shipped in 1957 as a component of the IBM 305 RAMAC system. 1301 used an array of heads, one per platter, moving as a single unit. Also in 1962, IBM introduced the model 1311 disk drive, which was about the size of a washing machine and stored two million characters on a removable disk pack. Users could buy additional packs and interchange them as needed, much like reels of magnetic tape.
Known as fixed-head or head-per-track disk drives they were very expensive and are no longer in production. In 1973, IBM introduced a new type of HDD code-named “Winchester”. Its primary distinguishing feature was that the disk heads were not withdrawn completely from the stack of disk platters when the drive was powered down. Instead, the heads were allowed to “land” on a special area of the disk surface upon spin-down, “taking off” again when the disk was later powered on. A few years later, designers were exploring the possibility that physically smaller platters might offer advantages.
As the 1980s began, HDDs were a rare and very expensive additional feature in PCs, but by the late 1980s their cost had been reduced to the point where they were standard on all but the cheapest computers. Most HDDs in the early 1980s were sold to PC end users as an external, add-on subsystem. External HDDs remained popular for much longer on the Apple Macintosh. Many Macintosh computers made between 1986 and 1998 featured a SCSI port on the back, making external expansion simple. NAND performance is improving faster than HDDs, and applications for HDDs are eroding. In 2018, the largest hard drive had a capacity of 15TB while the largest capacity SSD had a capacity of 30. 72TB or 100TB and HDDs are not expected to reach 100TB capacities until somewhere around 2025.
8-inches and below, were discontinued around 2010. The 2011 Thailand floods damaged the manufacturing plants and impacted hard disk drive cost adversely between 2011 and 2013. A modern HDD records data by magnetizing a thin film of ferromagnetic material on a disk. Sequential changes in the direction of magnetization represent binary data bits.
The data is read from the disk by detecting the transitions in magnetization. The platters are made from a non-magnetic material, usually aluminum alloy, glass, or ceramic. 20 nm in depth, with an outer layer of carbon for protection. The first HDDs spun at 1,200 rpm and, for many years, 3,600 rpm was the norm. Information is written to and read from a platter as it rotates past devices called read-and-write heads that are positioned to operate very close to the magnetic surface, with their flying height often in the range of tens of nanometers.
The read-and-write head is used to detect and modify the magnetization of the material passing immediately under it. In modern drives, there is one head for each magnetic platter surface on the spindle, mounted on a common arm. The arm is moved using a voice coil actuator or in some older designs a stepper motor. In 2004, a new concept was introduced to allow further increase of the data density in magnetic recording: the use of recording media consisting of coupled soft and hard magnetic layers. So-called exchange spring media magnetic storage technology, also known as exchange coupled composite media, allows good writability due to the write-assist nature of the soft layer.
HDD with disks and motor hub removed exposing copper colored stator coils surrounding a bearing in the center of the spindle motor. The voice coil itself is shaped rather like an arrowhead, and made of doubly coated copper magnet wire. The inner layer is insulation, and the outer is thermoplastic, which bonds the coil together after it is wound on a form, making it self-supporting. The HDD’s electronics control the movement of the actuator and the rotation of the disk, and perform reads and writes on demand from the disk controller. Feedback of the drive electronics is accomplished by means of special segments of the disk dedicated to servo feedback. LDPC codes enable performance close to the Shannon Limit and thus provide the highest storage density available.
The “No-ID Format”, developed by IBM in the mid-1990s, contains information about which sectors are bad and where remapped sectors have been located. As bit cell size decreases, more data can be put onto a single drive platter. The capacity of a hard disk drive, as reported by an operating system to the end user, is smaller than the amount stated by the manufacturer for several reasons: the operating system using some space, use of some space for data redundancy, and space use for file system structures. Also the difference in capacity reported in SI decimal prefixed units vs. Modern hard disk drives appear to their host controller as a contiguous set of logical blocks, and the gross drive capacity is calculated by multiplying the number of blocks by the block size. This information is available from the manufacturer’s product specification, and from the drive itself through use of operating system functions that invoke low-level drive commands. HDDs a certain number of sectors were reserved as spares, thereby reducing the capacity available to the operating system.