File:Computer Memory Hard Drive Disk HDD Storage Technology.jpg

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A hard disk drive ( english hard disk drive = HDD ), often referred to as hard drive or hard disk (abbreviated HD called) is a magnetic storage medium of computer technology , which writes data onto the surface of a rotating disk. This is the hard magnetic coating of the disk surface according to the information to be recorded magnetized . By the remanence is the storage of information. The readout of the information is carried out by scanning of the magnetization of the disk surface.

In contrast to sequentially addressable storage media such as magnetic tape or tape drives are the storage media direktadressierbaren (german direct access storage devices , DASD allocated) because you can directly access the data.

Recently, with comparable capacities are flash memory , called Solid State Drives (SSDs) and hybrid memory available (combination of SSD and hard disk drive), which over the same interfaces ( SATA , are, etc.) denotes addressed and simplified as "hard disks". SSDs are currently priced well over HDDs.

The term "hard drive" describes, first, that the magnetic disk is in contrast to the "Removable Disk" firmly connected to the drive, or the computer. Second, it corresponds to the English term "Hard Disk", which, unlike the flexible (English floppy ) disks in disk consists of rigid material.

The storage capacity of a hard disk is calculated from the size of a sector (512 bytes, 2048 bytes, or 4096 bytes) multiplied by the number of available sectors. The size of the first hard drives were measured in megabytes, from about 1997, in gigabytes, since about 2008, there are plates in the terabyte range.

The first hard disk drive IBM 350 of 1956 had an overall size of 24 ". Mid-1970 models came with a size of 8 ", which is also quite fast by the much more manageable and easier especially 5.25" were replaced hard drives. In between there were sizes of 14 "and 9".

The write head ( magnetic head ) of the writing finger, in principle, a tiny electromagnet , magnetized tiny areas of the disc surface and writes thus varying the data to the hard disk. Due to an air cushion, which is generated by the friction of the air to the rotating disk surface, the floating write (see -ground ). The flying height of 2000 was in the range of about 20 nm due to the short distance, the air inside the disk enclosure contain any impurities. With newer drives with perpendicular recording technology , this distance shrinks to 5 to 6 nm Currently announced plates (2011) permit with 1 TB / disk even at altitudes of up to 3 nm, so the signal by distance loss is not weakened too much. The production of hard disks are therefore like those of semiconductors in cleanrooms . The ground effect in this context is very useful for the maintenance of the correct altitude of the write head over the rotating disk.

In older disk write heads have been retracted in certain models of the plate stack. Later (1990, 2000) has been increasingly the park position indoors preferred. 2008 are found in both versions. For notebook drives the park position outside of the stack of plates provides extra protection against damage to the surface of the disks during transport (concussion) of the hard disk.

With modern disks is usually the true geometry , ie the number of sectors, heads and cylinders, which are managed by the hard disk controller, to the outside (that is for the computer or the hard disk drivers) are not visible.

The computer uses the hard drive works, then with a virtual disk, which has completely different geometry data. This explains why, for example, a hard disk, the real has only four heads, is seen by the computer with 255 heads. The reason for this virtual concept is that you wanted to overcome limitations of PC-compatible hardware. Today usual disks continue to share the plates in zones , whereby the number of blocks per track is equal to within a zone, but the change of the zone from the inside to the outside increases. The innermost zone has. Few blocks per track, the outermost zone, most blocks per track, so does the continuous transfer rate when changing zones from outside to inside.

The disk controller can hide bad blocks, and then expand a block from a reserve area. For the computer, it always looks as if all the defective blocks and usable.

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