Hard disk drive

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A Hard Disk unit (or HDD) is a storing device that stores digitally encrypted data on magnetic rotation surfaces called turntables.



HDDs were inserted for the first time as storing device in 1956 for IBM computers. Initially they were developed to be used as storing environments for general use computers.

In 1990 the necessity of a storing device of large dimensions that is trustful and independent led to the introduction of integrated systems like raids, NAS - Network Attached Storage and SAN - Storage Area Network. Taking into consideration the demands, in the 21 century the use of HDD extended to devices like video cameras, mobiles, digital audio / video players, PDAs and video game joysticks.

HDD technology

Recording data on HDD is realized trough the process of magnetizing a ferromagnetic disk called turntable. Registered data are written in binary. HDDs are built from an axis with one or more circular disks called turntables and one or more reading / writing heads. Turntables are made from a magnetic material, usually an aluminum alloy or glass, and are covered with a thin layer of magnetic material, usually 10 - 20 nm (nanometers) thick, with an exterior layer of carbon for protection.

Turntables are being rotated at very high speed. The information is written on the turntable wile rotating by read / write devices placed very close to the magnetic surface (dozens of nanometers). For each surface of the turntable there is a single read / write head mounted to a common arm. This arm moves the heads over turntables' surfaces on an arc distance (almost radial) to allow heads to access almost the entire surface of the turntables while rotating. The arm is moved using a coil-type actuator.

Storing capacities and access speeds

The use of sealed HDD in a single unit allows better tolerances than a floppy disk unit. As a consequence, HDD can store more data and they can access them faster.

  • Starting with April 2009 the largest HDD capacity is 2 TB;
  • Desktop HDDs can have a rotation speed between 5400 rpm ( revolutions per minute) and 7200 rpm and a transfer rate of 1 Gbit/s or higher.
  • The fastest HDD has 10000 or 15000 rpm and can reach over 1.6 Gbit/s transfer rate;
  • Mobile HDDs (laptop) are smaller and slower than desktop HDDs with a smaller storage capacity. Mobile HDDs have usually 5400 rpm up to 7200 rpm.

HDD shapes and sizes

HDD in different shapes and sizes

Before the PC and small computers era, HDDs had various sizes. Once the small computers sales increased was necessary a smaller HDD. Next are listed all available HDD sizes on the market:

  • 8 inch: 9.5 in × 4.624 in × 14.25 in (241.3 mm × 117.5 mm × 362 mm)
  • 5.25 inch: 5.75 in × 1.63 in × 8 in (146.1 mm × 41.4 mm × 203 mm)
  • 3.5 inch: 4 in × 1 in × 5.75 in (101.6 mm × 25.4 mm × 146 mm) = 376.77344 cm³
  • 2.5 inch: 2.75 in × 0.374–0.59 in × 3.945 in (69.85 mm × 9.5–15 mm × 100 mm) = 66.3575 cm³-104.775 cm³
  • 1.8 inch: 54 mm × 8 mm × 71 mm = 30.672 cm³
  • 1 inch: 42.8 mm × 5 mm × 36.4 mm
  • 0.85 inch: 24 mm × 5 mm × 32 mm

Other characteristics

Transfer rates

Starting with 2008 a desktop HDD typical unit of 7200 rpm has a data transfer rate "disk - to - buffer" of about 70 MB per second. A standard very used nowadays is the SATA standard of 3.0 Gbit/s which can send about 300 MB/s from buffer to computer.

Access time

Access time vary starting from under 2ms for server HDDs, 15 ms for mini HDDs and about 9 ms for typical desktop HDDs.

Noise level

Measured in dBA (decibels) the noise level is significant for certain applications like PVRs (personal video records), digital audio records and silence computers.

Shock resistance

Shock resistance is important especially for mobile devices. Some laptops include for HDD an active protection which lands the write/read heads before the impact if the system is dropped.

Connecting interfaces

SATA and external SATA HDD

HDDs are accessed trough a series of access interfaces, including Parallel ATA (P-ATA, also called IDE or EIDE), Serial ATA (SATA), SCSI, Serial Attached SCSI (SAS). Bridge circuits are used sometimes to connect disks to native interfaces which can't communicate trough IEEE 1394 (Firewire), USB and SCSI.

Usually ATA disks don't have problems regarding their controls but older models were incompatible and it was impossible to run two devices on the same cable in the master / slave configuration. This problem was solved in the mid 90s when ATA specifications were standardized.

Unlike HDD IDE, HDD Serial ATA (SATA) don't need to be configured as master / slave when combined with another device because they have their own I/O channels.

Shortcut Development Description
SASI Shugart Associates System Interface SCSI historical predecessor.
SCSI Small Computer System Interface Parallel comunication
SAS Serial Attached SCSI SCSI improvement, uses serial communication instead of the parallel one
ST-506 Seagate Technology Seagate historical interface.
ST-412 Seagate Technology Seagate historical interface (minor improvements of ST-506).
ESDI Enhanced Small Disk Interface Compatible with ST-412/506, but faster
ATA Advanced Technology Attachment ST-412/506/ESDI's successor
SATA Serial ATA ATA modification, uses serial communication instead of the parallel one