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radial path
A straight-line path from the center of a disk to the outer edge.

RAFF™
Rotary Acceleration Feed Forward. WD technology that maintains the highest possible data transfer performance in the high rotational vibration environments commonly found in servers and storage arrays.

RAID
Redundant array of independent disks. A grouping of hard drives in a single system to provide greater performance and data integrity.

RAID 0
RAID protocol in which data is striped across multiple hard drives, enabling the accelerated reading and recording of data by combining the work of two or more drives to increase performance. See also striping.

RAID 1
RAID protocol in which two copies of the data are instantaneously recorded – each on separate hard drives. RAID 1 ensures the protection of users’ data because in the event that one of the hard drives fails, the other hard drive(s) will continue to read and write data until the faulty hard drive is replaced and rebuilt to once again safely mirror the data. See also mirroring.

RAID 5
For systems with three or more drives, RAID 5 offers fast performance by striping data across all drives; data protection by dedicating a quarter of each drive to fault tolerance leaving three quarters of the system capacity available for data storage.

RAM
Random access memory. Memory that allows any storage location to be accessed randomly.

Ramp Load/Unload (LUL)
Ramp load parks the recording head off the media when the drive is idle and on spin up, maximizing available disk space and minimizing power usage, which results in lower heat and long-term drive reliability.

RE
RAID edition. A WD drive engineered to thrive in a high-intensity RAID system while still offering traditional desktop value.

read channel
The channel that performs data encoding and conversion that a drive requires to write computer generated information onto a magnetic medium and read back that information with a high degree of accuracy.

read verify
A data accuracy check performed by having a disk read data from a controller, which in turn checks for errors but does not pass data to the system.

read/write head
See head.

recoverable error
A read error that can be corrected by ECC or by re-reading data.

RLL
Run length limited. An encoding scheme used during write operations to facilitate reading that data.

RoHS
Restriction of Hazardous Substances. This compliance Directive 2002/95/EC of the European Parliament, which is effective in the EU beginning July 1, 2006, aims to protect human health and the environment by restricting the use of certain hazardous substances such as lead, mercury, hexavalent chromium, cadmium, polybrominated biphenyl flame retardants, and polybrominated diphenyl ether flame retardants in new equipment.

ROM
Read-only memory. An integrated circuit memory chip containing programs and data that the computer or host can read but cannot modify. A computer can read instructions from ROM but cannot store data in ROM.

rotational latency
The amount of delay in obtaining information from a disk due to disk rotation. For a disk rotating at 5400 RPM, the average rotational latency is 5.5 milliseconds. See also mechanical latency.

RPM
Rotations per minute. Also known as spindle speed. Rotational speed of a medium (disk). Hard drives typically spin at a constant speed. The lower the RPM, the higher the mechanical latency. Disk RPM is a critical component of drive performance, as it directly affects rotational latency.

RPS™
Reduced power spinup. The WD-optimized spinup feature specifically designed for the external hard drive and consumer electronics markets.