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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 .
The cost and capacity of storage arrays and tape libraries varies widely. As HDD prices have dropped, disk has become cheaper relative to tape drives and cartridges. For decades, the cost of a new LTO tape cartridge plus the tape drive required to make use of it, has been much greater than that of an equivalently sized HDD.
Hard disk drives store data in the magnetic polarization of small patches of the surface coating on a disk. The maximum areal density is defined by the size of the magnetic particles in the surface, as well as the size of the "head" used to read and write the data. In 1956 the first hard drive, the IBM 350, had an areal density of 2,000 bit/in 2.
Both have many parallel tracks across the width of the media and the read/write heads take time to switch between tracks and to scan within tracks. Different spots on the storage media take different amounts of time to access. For a hard disk this time is typically less than 10 ms, but tapes might take as much as 100 s. [citation needed]
A typical magnetic region on a hard-disk platter (as of 2006) is about 200–250 nanometers wide (in the radial direction of the platter) and extends about 25–30 nanometers in the down-track direction (the circumferential direction on the platter), [citation needed] corresponding to about 100 billion bits per square inch of disk area (15.5 ...
A block, a contiguous number of bytes, is the minimum unit of storage that is read from and written to a disk by a disk driver.The earliest disk drives had fixed block sizes (e.g. the IBM 350 disk storage unit (of the late 1950s) block size was 100 six-bit characters) but starting with the 1301 [8] IBM marketed subsystems that featured variable block sizes: a particular track could have blocks ...
[19] [failed verification] 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, e.g. the operating system using some space, use of some space for data redundancy, space use for file system structures.
Its speed is much faster than CD but not as fast as hard disk. The standard DVD-5 technology has a storage capacity of 4.7 GB per layer; most DVDs have a single layer but up to four layers are specified. Also DVD storage capacity changes with recording format.