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The first HDD [11] had an average seek time of about 600 ms. [12] and by the middle 1970s, HDDs were available with seek times of about 25 ms. [13]Some early PC drives used a stepper motor to move the heads, and as a result had seek times as slow as 80–120 ms, but this was quickly improved by voice coil type actuation in the 1980s, reducing seek times to around 20 ms.
That said, the price of high-capacity drives has fallen rapidly, and this is indeed an effect of density. The highest capacity drives use more platters, essentially individual hard drives within the case. As the density increases, the number of platters can be reduced, leading to lower costs. Hard drives are often measured in terms of cost per bit.
Drives use 700-oersted write heads for a density of 17,434 bpi. Extra-high density (ED) doubles the capacity over HD by using a barium ferrite coating and a special write head that allows the use of perpendicular recording. [1] [2] Triple density (TD) triples the capacity over ED by tripling the track density and improving other parameters.
Video hard drives, sometimes called "surveillance hard drives", are embedded into digital video recorders and provide a guaranteed streaming capacity, even in the face of read and write errors. [148] Drives embedded into automotive vehicles; they are typically built to resist larger amounts of shock and operate over a larger temperature range.
The exact capacity of a given drive model is usually slightly larger than the class designation. Practically all manufacturers of hard disk drives and flash-memory disk devices [5] [6] continue to define one gigabyte as 1 000 000 000 bytes, which is displayed on the packaging.
Compared to flash drives in same form factor, maximum rotating disk drive capacity is much smaller, [citation needed] with 100 TB available in 2018, [1] and 32 TB for 2.5-inch. [ 2 ] The disk drive size, such as 3.5-inch, is usually refers to the diameter of the disk platters .
For example, if three drives are arranged in RAID 3, this gives an array space efficiency of 1 − 1/n = 1 − 1/3 = 2/3 ≈ 67%; thus, if each drive in this example has a capacity of 250 GB, then the array has a total capacity of 750 GB but the capacity that is usable for data storage is only 500 GB.
A disk drive is a device implementing such a storage mechanism. Notable types are hard disk drives (HDD), containing one or more non-removable rigid platters; the floppy disk drive (FDD) and its removable floppy disk; and various optical disc drives (ODD) and associated optical disc media.