Data Recovery and Hard Disk Drive Glossary of Terms
DATA RECOVERY TERMS
Embedded servo or wedge servo is a type of servo configuration used on today's hard disk drives This is a major cause of catastrophic hard drive failure and one of the main reasons data may not be recoverable from a hard drive. Furthermore it is the very reason that disk drive platters cannot be removed and put into another hard drive.
Embedded servo systems embed the feedback signals for the read/write head positioner (usually a voice coil motor) inside gaps or wedges in the data tracks of the disk. These are sometimes referred to as servo markers. This setup allows the entire set of platters to be used, instead of having to reserve one or two surfaces for the servo's use (dedicated servo), which makes more space for data available on the drive. Embedded servo was originally developed in the 1970s, and started to appear on mass-market hard drives for personal computers in the late 1980s.
As with dedicated-servo drives, the control signals are written at the factory using a special device called a servo writer, and cannot usually be regenerated in the field. Hard Disk Drives with errors in the servo areas are considered to be badly damaged and the data is usually not recoverable from them.
Head Crash is a term used to refer to the R/W head or heads inside a hard drive touching or hitting the platters or disks. Normally when a hard drive is running the heads float a few microns above the media. A tiny particle of dirt or debris can cause the head to bounce against the media. When it does touch the media it makes marks or scratches on the surface when can cause data loss depending upon how severe the head crash is. Following a head crash, particles of material scraped free of the drive surface greatly increase the chances of further head crashes or damage to the platters. When a disk drive starts making unusual sounds it should be turned off immediately. Continued running of a disk drive suffering from a head crash will only cause further damage. A hard drive should never be opened up outside of a clean room.
PCBA - The "Printed Circuit Board Assembly" is the electronics board attached to the disk drive. This is a common cause of hard drive failure particularly when there is a power surge. It should only be replaced by a professional data recovery company. A PCBA failure can sometimes lead to internal damage of the hard disk drive.
PLATTER - The disk inside the drive that spins at a high rate of speed and is used as the recording surface of computer data files in the form of magnetic north and south poles. It is made of aluminum or glass and has a metal oxide coating that is "sputtered" on. When the disk becomes scratched or marked by the heads data loss can occur.
READ/WRITE HEADS - Hard Disk read/write heads are the mechanisms that read data from or write data to hard disk drives . R/W heads continue to change as the hard disk drive evolves. There have been numerous changes over the years. In a hard drive, the heads fly above the disk surface with clearance of as little as 3 nanometers..
STICKTION - A fluorocarbon that builds up between a Read/Write head and platter. The result is the disk drive may not spin up.
HARD DISK DRIVE TERMS - DISK DRIVE INTERFACES
FD-400 Interface - Pertec Peripherals came up with this 8 inch floppy interface around 1974 and it is the predecessor to Shugart Associates 8 inch (50 pin) floppy interface. This drive utilized a 44-pin cable with a edge card type connector that supplied power as well as data and control signals. This drive is actually still in use today primarily on flight simulators and CNC milling machines.
SA-400 Interface - Shugart Associates designed the SA-400 floppy disk drive in 1978 which was the first 5.25" floppy drive to gain wide acceptance. This drive utilized a 34-pin cable that is still used in floppy drives today. The interface was later modified for hard drives and this modified version became the ST-506 interface. Although the pin outs of this interface have changed slightly over the years it is still the standard for 5.25" and 3.5" floppy disk drives.
IPI (Intelligent Peripheral Interface) - This hard drive interface was used on 8" and 14" mainframe and minicomputer disk drives in the 1960's and 1970's.It can be found on disk drives manufactured from Control Data Corp. which is now part of Seagate Technology.
SMD (Storage Module Device) - This hard drive interface replaced IPI and was used on 8" and 14" mainframe and minicomputer disk drives until it was replaced by SCSI. It can be found on disk drives manufactured from IBM,CDC and Pertec. Later an extended version called XSMD came out as an extended version of SMD.
ST506/412 - This interface was developed by Seagate Technology (originally Shugart) in 1980 for use with their ST-506 hard drive (5 megabytes). It was later revised in 1981 with a feature called "buffered seek" for their ST-412 (a 10 megabytes version). Due to its design limitations you would not see this interface on drives larger than 140 megabytes. The interface consists of two cables, a 34 pin and a 20 pin used for drive control and data transfer. This interface was most commonly used in 80286 computers.
ESDI (Enhanced Small Device Interface) - An ad hoc group of controller and device manufacturers (led by Maxtor Corporation) met to develop a standard that would increase the data capacity and speed of the existing ST506/ST412 interface. The first standard The Enhanced Small Disk Interface was released in 1983 but then it was decided to merge this interface with the Enhanced Tape interface into one standard in October of 1983. Although the cabling of this interface is the same as ST506 it performs a lot better error checking and the encoding and decoding is performed on the drive and not the controller making for a faster transfer rate than its predecessor. This interface was used on disk drives at the time in the range of 138mb to 676mb.
SCSI (Small Computer System Interface) - This interface originated as the SASI (Shugart Associates System Interface) in 1979. It was one of several disk interfaces of that time that worked at a logical level instead of the widely accepted device level. Working at a logical level allowed for a stable interface while the disk devices could change rapidly. In 1980 Shugart Associates attempted to replace the IPI (Intelligent Peripheral Interface) through the standards committee, but were not able to due to limited industry acceptance. NCR added features to Shugart's original interface and in 1982 the standards committee decided to start a project for SCSI which was to be based on SASI. During the project, optical WORM commands were added, no longer limiting SCSI to disks. In 1986 ANSI approved SCSI as a standard. Connecting via a 50 -pin ribbon cable, up to seven "different" devices can be connected at the same time. This includes hard drives, tape drives, floppy drives and even printers.
IDE (Integrated Drive Electronics) - This interface originated in 1988 when a number of peripheral suppliers formed the Common Access Method Committee to push an industry-wide effort of adopting a standard software interface for SCSI peripherals. Part of their goal at that time was to specify what is now known as the ATA (AT Attachment Interface) which would allow an interface to be designed into the the AT -compatible motherboards of that time. The ATA interface usually was not mentioned at the time , it was encompassed in the IDE term. ATA referred to the interface itself and IDE to the hard drive. Hard drives used with this type of interface are "intelligent" devices that have most of the controller functions built into the drive circuit board. These drives have a 40 -pin connector that plug directly into the motherboard.
ATA (AT Attachment Interface) - ATA defines a universally agreed upon register set and a 40 pin connector and its associated signals. This is the AT bus or IDE interface.
XTA (XT Attachment) - This was an implementation of the ATA Interface that used an integrated 8 bit XT controller during the late 1980's Although rarely used one computer manufacturer of the time that did use it was Vendex. Miniscribe make a drive called the MS8425XT (20Mb.) at that time.
ATA-2 - This was an upgrade to the ATA interface standard to provide for greatly increased
EIDE (Enhanced IDE) and FAST-ATA - This is an enhanced version of the ATA-2 standard as marketed by disk drive manufacturers such as Seagate/Quantum (FAST-ATA) and Western Digital (EIDE).
Ultra ATA/100 - Ultra ATA/100 is the latest generation of the parallel ATA interface. ATA, also known as IDE, which is the most common interface for desktops and workstations. As ATA disc drives have become faster internally, the need has arisen for faster interface or external "burst" transfer rates. Recent examples of this include Ultra ATA/33, Ultra ATA/66, and now Ultra ATA/100. The numeric portion of these interfaces denotes the maximum burst transfer rate of the interface in Megabytes per second.
SATA - Serial ATA is the latest computer bus technology used to transfer data to and from a hard disk drive. It uses a 7 pin data cable and 15 pin power cable The relative simplicity of a serial link allow both the use of longer drive cables (39" vs. 18" for PATA) and an easier transition path to higher speeds. Currently data transfer rates are 300 mb/s.
DISK DRIVE RECORDING: DATA ENCODING SCHEMES
FM ENCODING (Frequency Modulation Encoding) - This is an outdated encoding scheme that was used in 8 inch floppy disk drives during the 1970's. The problem with it was that it used up half of the disk space for timing signals used in the encoding process. Later (in the 1980's) the technology was refined and replaced with a new standard called MFM encoding.
MFM ENCODING (Modified Frequency Modulation Encoding) - This is an encoding scheme was was an enhancement to FM. Basically this scheme converts the digital bits from the computer into a pattern of magnetic changes or "flux reversals" that are stored on the hard drive. MFM does away with the need for timing signals thus it is more efficient than FM. It was widely used in hard drives in the 1980's and is still in use today in floppy drives.
RLL ENCODING (Run Length Limited) - This is an encoding scheme that reduces the amount of data-checking information that is stored and thus requires less flux reversals for a given amount of data. The logic circuitry is more more complicated than MFM but allows much more data to be recorded on the disk drive. In RLL 2,7 the "run length" of zeros is limited to 7. The codes are chosen so that the sequences of zeros in the codes always range from 2 to 7. This allows for a 50% increase in disk space over MFM.
ARLL ENCODING (Advanced Run Length Limited) - This is an advanced version of RLL that has a run length of zeros from 3 to 9 which even further increases disk space up to (100% over MFM).
