Interview Question : What is Hard Disk?


A hard disk is part of a unit, often called a “disk drive,” “hard drive,” or “hard disk drive (HDD),” that stores and provides relatively quick access to large amounts of data on an electromagnetically charged surface or set of surfaces. Today’s computers typically come with a hard disk that contains several billion bytes (gigabytes) of storage.

A Hard disk can also be defined as:

  1. a rigid (“hard”) non-removable magnetic disk with a large data storage capacity.
  2. a data storage device used for storing and retrieving digital information using one or more rigid (“hard”) rapidly rotating disks (platters) coated with magnetic material.
  3. A magnetic disk on which you can store computer data. The term hard is used to distinguish it from a soft, or floppy disk. Hard disks hold more data and are faster than floppy disks.

Extra Information

A hard disk is really a set of stacked “disks,” each of which, like phonograph records, has data recorded electromagnetically in concentric circles or “tracks” on the disk. A “head” (something like a phonograph arm but in a relatively fixed position) records (writes) or reads the information on the tracks. Two heads, one on each side of a disk, read or write the data as the disk spins. Each read or write operation requires that data be located, which is an operation called a “seek.” (Data already in a disk cache, however, will be located more quickly.)

A hard disk/drive unit comes with a set rotation speed varying from 4500 to 7200 rpm. Disk access time is measured in milliseconds. Although the physical location can be identified with cylinder, track, and sector locations, these are actually mapped to a logical block address (LBA) that works with the larger address range on today’s hard disks.

To know more regarding the terms follow the post about Difference between Disc and Disk click here.


Quantum optical hard drive breakthrough

This image shows quantum information being written on to the nuclear spins of a europium ion. Credit: Solid State Spectroscopy Group, ANU
This image shows quantum information being written on to the nuclear spins of a europium ion.
Credit: Solid State Spectroscopy Group, ANU

Scientists developing a prototype quantum hard drive have improved storage time by a factor of more than 100.

The team’s record storage time of six hours is a major step towards a secure worldwide data encryption network based on quantum information, which could be used for banking transactions and personal emails.

“We believe it will soon be possible to distribute quantum information between any two points on the globe,” said lead author Manjin Zhong, from the Research School of Physics and Engineering (RSPE) at The Australian National University (ANU).

“Quantum states are very fragile and normally collapse in milliseconds. Our long storage times have the potential to revolutionise the transmission of quantum information.”

Quantum information promises unbreakable encryption because quantum particles such as photons of light can be created in a way that intrinsically links them. Interactions with either of these entangled particles affect the other, no matter how far they are separated.

The team of physicists at ANU and the University of Otago stored quantum information in atoms of the rare earth element europium embedded in a crystal.

Their solid-state technique is a promising alternative to using laser beams in optical fibres, an approach which is currently used to create quantum networks around 100 kilometres long.

“Our storage times are now so long that it means people need to rethink what is the best way to distribute quantum data,” Ms Zhong said.

“Even transporting our crystals at pedestrian speeds we have less loss than laser systems for a given distance.”

“We can now imagine storing entangled light in separate crystals and then transporting them to different parts of the network thousands of kilometres apart. So, we are thinking of our crystals as portable optical hard drives for quantum entanglement.”

After writing a quantum state onto the nuclear spin of the europium using laser light, the team subjected the crystal to a combination of a fixed and oscillating magnetic fields to preserve the fragile quantum information.

“The two fields isolate the europium spins and prevent the quantum information leaking away,” said Dr Jevon Longdell of the University of Otago.

The ANU group is also excited about the fundamental tests of quantum mechanics that a quantum optical hard drive will enable.

“We have never before had the possibility to explore quantum entanglement over such long distances,” said Associate Professor Matthew Sellars, leader of the research team.

“We should always be looking to test whether our theories match up with reality. Maybe in this new regime our theory of quantum mechanics breaks.”

Story Source:

The above story is based on materials provided by Australian National University.Note: Materials may be edited for content and length.

Journal Reference:

  1. Manjin Zhong, Morgan P. Hedges, Rose L. Ahlefeldt, John G. Bartholomew, Sarah E. Beavan, Sven M. Wittig, Jevon J. Longdell, Matthew J. Sellars. Optically addressable nuclear spins in a solid with a six-hour coherence time. Nature, 2015; 517 (7533): 177 DOI: 10.1038/nature14025