HDD User's Guide --- Glossary

Adsorption

When the HDD is left unused, the head may stick to the disk so the spindle motor does not start. This state is called adsorption.
Adsorption is likely to occur when the HDD is exposed, over a long period of time, to humidity in excess of the limit specified in the device specifications. Adsorption is also easily caused by condensation.

CSS (Contact Start Stop)

While the disk is stopped, the head contacts the disk. When the disk begins rotation, the head floats. When the disk stops rotating, the head contacts the disk again. This operation mode is called the Contact Start Stop (CSS) mode. Most current HDDs use the CSS mode. In CSS mode, the head is put in an unstable floating state and slides and wears the disk surface each time the disk starts or stops rotating.
Fujitsu HDDs are designed to perform CSS in an area (CSS zone) separate from the data area so as to prevent damage to the data area during CSS.
The head slides over the disk surface during CSS. Therefore, disk reliability degrades as the CSS frequency increases.
However, CSS also has a positive effect in that it removes dust stuck to the head. It is not always good to decrease the CSS frequency.
For maintenance of HDD reliability, it is recommended that 24-hour continuous running systems execute CSS at least once a week.

DE (Disk Enclosure)

The Disk Enclosure (DE) is also called the Head Disk Assembly (HDA). The DE is a part of the HDD and consists of a spindle motor, magnetic disk, voice coil motor, and head actuator.
Since the head floats very low over the disk surface while the HDD is operating, the presence of dust must be prevented. Therefore, the DE is sealed to shut out external air.

Head crash

In the HDD, the magnetic head floats very low over the disk surface to read or write data.
Dust or liquid entering and adhering to the head surface disturbs the floating head and causes contact between the head and disk. Such a foreign substance adhering between the head and disk may be naturally removed by the head, and the head may thus be recovered to its stable floating state, so no problem occurs.
However, contact between the head and disk may generate abrasive powder and increase the amounts of adhered substances. The adhered substance may solidify as a result of a temperature rise due to friction. With increasing speeds these factors worsen and may finally damage the head or disk. When the head or disk is thus damaged, a head crash has occurred.
A head crash roughens the disk and head surfaces preventing the head from floating normally. It may also damage the magnetic recording layer and disable data access.
To help prevent head crashes, our HDDs have sealed DEs to prevent dust from entering. A circulation filter in the DE collects dust generated within the DE. Note, however, that even our HDDs may have a head crash if water droplets or dust is generated inside the DE by condensation or a shock greater than the specified level.

MTBF (Mean Time Between Failures)

MTBF is the mean value of the time between consecutive failures under specified conditions for a specified period in the life of an HDD.