Glossary of Hard Disk Drive Terminology (Letter N)
Network Computer
A kind of computer that contains limited data storage capacity and is used to communicate with a central data storage facility such as a server or RAID system.
Hard Disk Repair
Hard Disk Repair, hd repair, hdd repair, Hard Disk Repair Technologies
Glossary of Hard Disk Drive Terminology (Letter M)
Magnetic flux
The pattern of magnetic pole directions of the bits written on the disk.
Manufacturing Yield
The portion of unit production of a manufacturing process that is usable, saleable product; usually expressed as a percent of total output of that product.
Master
The first drive in a dual drive combination. A master drive by itself (with no slave) is called a single drive.
Media
In hard drives, the disks and their magnetic coatings; sometimes refers to the coating material alone.
MB (Megabyte)
Western Digital defines a megabyte as 1,000,000 (one million) bytes.
Mechanical Latencies
Include both seek time and rotational latency. Mechanical latencies are the main hindrance to higher performance in hard drives. The time delays of mechanical latencies are one hundred times higher than electronic (non-mechanical) latencies associated with the transferring of data. See also Seek Time, Rotational Latency.
Memory
A device or storage system capable of storing and retrieving data.
MFM (Multiple Frequency Modulation)
A method of encoding analog signals into magnetic pulses or bits.
MR Heads (Magneto-resistive Heads)
MR heads were developed to increase areal density and improve drive performance. MR heads use separate read and write elements, as opposed to traditional inductive thin-film read-write heads. MR heads use an inductive element for writing data, and a separate magneto-resistive element for reading information. The read element has a magnetically sensitive material that detects data recorded on the magnetic disk surface. MR head construction results in a stronger signal than that produced by inductive thin-film read-write heads, which permits it to read higher areal density data. Since the magneto-resistive element can only read data, a conventional thin-film inductive element writes data to the disk.
MTBF (Mean Time Between Failures)
Average time (expressed in hours) that a component works without failure. It is calculated by dividing the total number of operating hours observed by the total number of failures. Also, the length of time a user may reasonably expect a device or system to work before an incapacitating fault occurs.
MTTR (Mean Time to Repair)
The average time it takes to repair a drive in the field. In the field, only major subassemblies are changed (the PCB, sealed housing, etc.), excluding component level repairs as these are not performed in the field.
Multi-media
A simultaneous presentation of data in more than one form, such as by means of both visual and audio.
Multi-user
In information technology, a system that enables more than one user to access data at the same time.
Glossary of Hard Disk Drive Terminology (Letter L)
Landing Zone
The heads move to this location on the inner portion of the disk when commanded, or when the power has been turned off. User data is not stored in this area of the disk.
Laser Textured Media
Laser textured disks minimize the wear and friction on a hard drive. The precision and consistency of the laser zone texturing process is a contributor to the robustness of our Western Digital hard drives.
Latency
The period of time that the read/write heads wait for the disk to rotate to the correct position to access the desired data. For a disk rotating at 5200 RPM, the average latency is 5.8 milliseconds; or, the average time delay between the head arriving on track and the data rotating to the head. (Calculated as one-half the revolution period.)
Local Area Network (LAN)
A system in which computer users in the same company or organization are linked to each other and often to centrally-stored collections of data in LAN servers.
Logical Address
A storage location address that may not describe the physical location; instead, it used as a means to request information from a controller. The controller converts the request from a logical to a physical address that is able to retrieve the data from an actual physical location on the storage device.
LBA (Logical Block Addressing)
A method of addressing the sectors on a drive. Addresses the sectors on the drive as a single group of logical block numbers instead of cylinder, head and sector addresses. It allows for accessing larger drives than is normally possible with CHS addressing.
Logical Drive
A logical drive is a section of the hard disk that appears to be a separate drive in a directory structure. You create logical drives on the extended partition of a hard disk. While 26 letters exist for logical drives, the first three are reserved. A and B are reserved for floppy disk drives, and C is reserved for the first primary DOS partition. Therefore, you can create up to 23 logical drives on your extended partition. Logical drives are usually used to group directories and files.
Logistics Model
The systems by which a company organizes the physical distribution of its products. A hard drive manufacturer’s model might include portions to OEM customers, to distributors, to retail chains or to all of these.
Low-level formatting
The process of creating sectors on the disk surface; this permits the operating system to use the regions needed to create the file structure. Also called initialization. Low-level formatting is performed at the Western Digital factory. There is no need for you to low-level format a Western Digital drive.
Low profile (LP)
Standard 3.5-inch hard drives are available in heights of 1.0-inch and 1.6-inches. Low-profile hard drives measure 1.0-inches in height.
Glossary of Hard Disk Drive Terminology (Letter K)
Kilobyte (KB)
Usually, this is a unit of 1000 bytes. In the case of computer memory, which is partitioned into sizes that are a power of two, a kilobyte is equal to 2 10 or 1024 bytes.
Glossary of Hard Disk Drive Terminology (Letter J)
Jumper
In EIDE drives, a jumper is an electrically-conductive component that you place over pairs of pins that extend from the circuit board on the hard drive jumper block to connect them electronically. For example, a jumper is one way to designate a hard drive as master or slave. The jumper block is located next to the 40-pin connector on the hard drive.
Just-in-time (JIT)
A production and inventory control process in which components and materials are delivered to an assembly point as needed. This process is used in Western Digital manufacturing facilities and in most of the company’s customer plants.
Glossary of Hard Disk Drive Terminology (Letter I)
IDE (Integrated Drive Electronics)
A type of drive where the interface controller electronics are incorporated into the design of the hard drive rather than as a separate controller.
Index Pulse Signal
A digital pulse signal indicating the beginning of a disk revolution. An embedded servo pattern or other prerecorded information is present on the disk following index.
Initiator
A device in control of the SCSI bus that sends commands to a target. Most SCSI devices have a fixed role as an initiator or a target; however, some devices can assume both roles.
Initialization
See low-level formatting.
Input
The incoming data that the computer processes, such as commands issued by the user.
Input/output (I/O)
An operation or device that allows input and output.
Interface
A hardware or software protocol that handles the exchange of data between the device and the computer; the most common ones are AT (also known as IDE) and SCSI. (See AT and SCSI.)
Interface controller
The chip or circuit that translates computer data and commands into a form suitable for use by the hard drive and controls the transfer of data between the buffer and the host. (See disk controller and disk drive controller.)
Interleave
The arrangement of sectors on a track.
Interrupt
A signal sent by a subsystem to the CPU that signifies a process has either completed or could not be completed.
ISA
Industry Standard Architecture. The standard 16-bit AT bus designed by IBM for the PC/AT system. ISA was the only industry standard bus for PCs until the recent release of MCA (MicroChannel Architecture), EISA (Extended Industry Standard Architecture), and PCI (Peripheral Component Interconnect).
Glossary of Hard Disk Drive Terminology (Letter H)
Half-Duplex
A communications protocol that permits transmission in both directions but in only one direction at a time.
Half-height Drives
Standard 3.5-inch hard drives are available in heights of 1.0-inch and 1.6-inches. Half-height drives measure 1.6-inches in height.
Hard Drive
An electromechanical device used for information storage and retrieval, incorporating one or more rotating disks on which data is recorded, stored and read magnetically. Western Digital’s principal product.
Hard Drive Industry
The combined manufacturers of hard drives. In the United States, the industry is led by IBM, Maxtor, Seagate, Quantum and Western Digital.
Hard Error
An error that is repeatable every time the same area on a disk is accessed.
Hard Sectored
A technique that uses a digital signal to indicate the beginning of a sector on a track.
Head
The minute electromagnetic coil and metal pole which write and read back magnetic patterns on the disk. Also known as a read/write head. A drive with several disk surfaces or platters will have a separate head for each data surface. See also MR Head.
Head Actuator
A motor that moves the head stack assembly in a hard drive to align read/write heads with magnetic tracks on the disks.
Head Crash
Refers to the damage incurred to a read/write head when the head comes into contact with the disk surface. A head crash might be caused by severe shock, dust, fingerprints, or smoke, and can cause damage to the surface of the disk and/or the head.
Head Disk Assembly (HDA)
The mechanical components of a hard drive, including the disks, heads, spindle motor and actuator.
Head Loading Zone
An area on the disk specifically reserved for the heads to use when taking off or landing when power to the drive is turned on or off. No data storage occurs in the head loading zone.
Head Stack Assembly
The electromechanical mechanism containing read/write heads and their supporting devices.
Headerless Format
The lack of a header or ID fields (track format). This enables greater format efficiency and increased user capacity.
High-end Market
The enterprise market.
High-Level Format
A high-level format must be performed (with EZ-Drive or the Format command) on your new Western Digital hard drive before you can use it. Formatting erases all the information on a hard drive and it sets up the file system needed for storing and retrieving files.
Host
The computer that other computers and peripherals connect to. See also initiator.
Host Adapter
A plug-in board that acts as the interface between a computer system bus and the disk drive.
Host Interface
The point at which the host and the drive are connected to each other.
Host Transfer Rate
Speed at which the host computer can transfer data across the SCSI interface; or, the speed at which the host computer can transfer data across the EIDE interface. Processor Input/Output (PIO) modes and Direct Memory Access (DMA) modes are defined in the ATA-4 industry specifications for the EIDE interface.
Glossary of Hard Disk Drive Terminology (Letter G)
GB (Gigabyte)
Western Digital defines a gigabyte as 1,000,000,000 (one billion) bytes or 1000 (one thousand) Megabytes.
Giant MR (GMR)
An advanced form of magnetoresistive head technology in which IBM is believed to hold the greatest product advancements.
Glossary of Hard Disk Drive Terminology (Letter F)
FAT (File Allocation Table)
A data table stored at the beginning of each partition on the disk that is used by the operating system to determine which sectors are allocated to each file and in what order.
Fdisk
A software utility used to partition a hard drive. This utility is included with DOS and Windows 95 operating systems.
Fetch
The process of retrieving data.
Fibre Channel (FC)
The general name given to an integrated set of standards being developed by an ANSI-approved X3 group. This set of standards defines new protocols for flexible information transfer. Fibre channel supports three topologies: point-to-point, arbitrated loop, and fabric.
Fibre Channel Arbitrated Loop (FC-AL)
A subset of fibre channel network systems interconnection. A serial storage interface designed to meet the needs of high-end applications.
Fiscal Periods
Three-month segments of a fiscal year. Western Digital, with a June 30 fiscal year, has fiscal quarters ending on the last days of September, December, March and June.
Firmware
Permanent instructions and data programmed directly into the circuitry of read-onlymemory for controlling the operation of the computer.
FIT (Functional Integrity Testing)
A suite of tests Western Digital performs on all its drive products to ensure compatibility with different hosts, operating systems, adapters, application programs, and peripherals. This testing must be performed before the product can be released to manufacturing.
Flow Control
In PIO transfers, the ability of an EIDE drive to control the speed at which the host transfers data to or from the drive by using the IORDY signal. The host temporarily stops transferring data whenever the drive deasserts the IORDY signal. When the drive reasserts the IORDY signal, the host continues the data transfer.
Format
A process that prepares a hard drive to store data. Low-level formatting sets up the locations of sectors so user data can be stored in them. Western Digital hard drives are low-level formatted at the factory and therefore do not need to be low-level formatted by the end user. You need to perform a high-level format (with EZ-Drive or the Format command) on your new Western Digital hard drive before you can use it. Formatting erases all the information on a hard drive and it sets up the file system needed for storing and retrieving files.
Formatted Capacity
The actual capacity available to store data in a mass storage device. The formatted capacity is the gross capacity minus the capacity taken up by the overhead data required for formatting the media.
Form Factor
The industry standard that defines the physical and external dimensions of a particular device.
Full-Duplex
A communication protocol that permits simultaneous transmission in both directions.
Solve Disk Imaging Problems (Part 3)
Customizing Imaging Algorithms
Consider the following conflicting factors involved in disk imaging:
§§ A high number of read operations on a failed drive increase the chances of recovering all the data, and decrease the number of probable errors in that data.
§§ Intensive read operations increase the rate of disk degradation and increase the chance of catastrophic drive failure during the imaging process.
§§ Imaging a drive can take a long time (for example, one to two weeks) depending on the intensity of the read operations. Customers with time-sensitive needs may prefer to rebuild data themselves rather than wait for recovered data.
Clearly these points suggest the idea of an imaging algorithm that maximizes the probable data recovered for a given total read activity, taking into account the rate of disk degradation and the probability of catastrophic drive failure.
However, no universal algorithm exists. A good imaging procedure depends on such things as the nature of the drive problem, and the characteristics of the vendor-specific drive firmware. Moreover, a client is often interested in a small number of files on a drive and is willing to sacrifice the others to maximize the possibility of recovering those few files.To meet these concerns, the judgment of the imaging tool operator comes into play.
Drive imaging can consist of multiple read passes. A pass is one attempt to read the entire drive, although problem sectors may be read several times on a pass or not at all, depending on the configuration. The conflicting considerations mentioned above suggest that different algorithms, or at least different parameter values, should be used on each pass.
The first pass could be configured to read only error-free sectors. There is a fair possibility that the important files can be recovered faster in this way in just one pass. Moreover, this pass will not be read-intensive since only good sectors are read and the more intensive multiple reads needed to read problem sectors are avoided. This configuration reduces the chances of degrading the disk further during the pass (including the chances of catastrophic drive failure) while having a good chance at recovering much of the data.
Second and subsequent passes can then incrementally intensify the read processes, with the knowledge that the easily-readable data have already been imaged and are safe. For instance, the second pass may attempt multiple reads of sectors with the UNC or AMNF error (Figure 2). Sectors with the IDNF error are a less promising case, since the header could not be read and hence the sector could not be found. However, even in this case multiple attempts at reading the header might result in a success, leading to the data being read. Successful data recovery of sectors with different errors depends on the drive vendor. For example, drives from some vendors have a good recovery rate with sectors with the IDNF error, while others have virtually no recovery. Prior experience comes into play here, and the software should be configurable to allow different read commands and a varying number of reread attempts after encountering a specific error (UNC, AMNF, IDNF, or ABRT).
Drive firmware often has vendor-specific error-handling routines of its own that cannot be accessed directly by the system. While you may want to minimize drive activity to speed up imaging and prevent further degradation, drive firmware increases that activity and slows down the process when faced with read instability. To minimize drive activity, imaging software must implement a sector read timeout, which is a user-specified time before a reset command is sent to the drive to stop processing the current sector.
For example, you notice that good sectors are read in 10 ms. If this is a first pass, and your policy is to skip problem sectors at this point, the read timeout value might be 20 ms. If 20 ms have elapsed and the data has not yet been read, the sector is clearly corrupted in one way or another and the drive firmware has invoked its own error-handling routines. In other words, a sector read timeout can be used to identify problem sectors. If the read timeout is reached, the imaging software notes the sector and sends a reset command. After the drive cancels reading the current sector, the read process continues at the next sector.
By noting the sectors that timeout, the software can build up a map of problem sectors. The imaging algorithm can use this information during subsequent read passes.
In all cases the following parameters should be configurable:
- Type of sectors read during this pass
- Type of read command to apply to a sector
- Number of read attempts
- Number of sectors read per block
- Sector read timeout value
- Drive ready timeout value
- Error-handling algorithm for problem sectors
Other parameters may also be configurable but this list identifies the most critical ones.
Imaging Hardware Minimizes Damage
In addition to the software described above, data recovery professionals also need specialized hardware to perform imaging in the presence of read instability. Drive firmware is often unstable in the presence of read instability, which may cause the drive to stop responding. To resolve this issue, the imaging system must have the ability to control the IDE reset line if the drive becomes unresponsive to software commands. Since modern computers are equipped with ATA controllers that do not have ability to control the IDE reset line, this functionality must be implemented with a specialized hardware. In cases where a drive does not even respond to a hardware reset, the hardware should also be able to repower the drive to facilitate a reset.
If the system software cannot deal with an unresponsive hard drive, it will also stop responding, requiring you to perform a manual reboot of the system each time in order to continue the imaging process. This issue is another reason for the imaging software to bypass the system software.
Both of these reset methods must be implemented by hardware but should be under software control. They could be activated by a drive ready timeout. Under normal circumstances the read timeout sends a software reset command to the drive as necessary. If this procedure fails and the drive ready timeout value is reached, the software directs the hardware to send a hardware reset, or to repower the drive. A software reset is least taxing on repower method is most taxing. A software reset minimizes drive activity while reading problem sectors, which reduces additional wear. A hardware reset or the repower method deals with an unresponsive hard drive.
Moreover, because reset methods are under software control via the user-configurable timeouts, the process is faster and there is no need for constant user supervision.
The drive ready timeout can also reduce the chances of drive self-destruction due to head-clicks, which is a major danger in drives with read instability. Head-clicks are essentially a firmware exception in which repeated improper head motion occurs, usually of large amplitude leading to rapid drive self-destruction. Head-clicks render the drive unresponsive and thus the drive ready timeout is reached and the software shuts the drive down, hopefully before damage has occurred. A useful addition to an imaging tool is the ability to detect head-clicks directly, so it can power down the drive immediately without waiting for a timeout, thus virtually eliminating the chances of drive loss.