Anatomy of a Hard Drive
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A-Z of Data Recovery
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A-Z of Data Recovery
A
ADF - Data file used by the ACT CRM application.
Areal Density - The number of bits that can be stored on a square inch of disk. Historically, areal density increased at an annual compound growth rate of around 25% throughout the 1970s and 1980s. In the 1990's, this increased to 60% per year thanks largely to the development of magnetoresistive heads replacing inductive heads.
AVI file - Audio Video Interleave. Audio and video playback format developed my Microsoft in 1992. For playback, an AVI file relies on codecs which are compression-decompression algorithms. Popular codecs include MPEG-4, MJPEG DivX, Xvid and HuffYUV.
ATA Password - (a.k.a. HDD password) A password stored in a hard drive's controller which makes a drive inaccessible unless a valid password is provided. While an ATA password (if the host system's BIOS allows it) offers some degree of protection from unauthorised access to your hard drive, it is still does not offer the same protection as whole-disk encryption.
B
Bad Sector - A bad sector is an area of a hard drive that cannot be used due to damage. Much to many user's surprise, hard drives have always left the production line with pre-existing bad sectors. Even in the days of MFM drives (1980's), new drives came with a sticker listing of all bad sectors found during quality control. Today's hard disk leaves the production line with bad sectors marked on its P-List. To compensate for bad sectors, each hard disk holds a certain amount of spare clusters. If during the read/write process, a failure does occur in a drive cluster, this cluster will be marked as bad and a spare cluster will be used in lieu. The end user will unaware of this process. However, if the number of bad sectors on a drive outstrips the number of spare clusters and the user sometimes gets notified of these errors. For example, the Windows user will begin to see sporadic "bad block" messages in their Event Viewer, or will begin to notice the Checkdisk (chkdsk) utility appear during the Windows boot-up process or they will get a notification from SMART.
Bearing Seizure - Bearings are needed for the drive's spindle to run smoothly and efficiently. Occasionally, these bearings seize and the spindle jams. One symptom of seized bearings is when a hard drive is unable to spin-up but makes an audible beeping or buzzing noise. Seized spindles are a common occurrence on external drives which have incurred shock damage.
Blue Screen - (a.k.a stop error or Blue Screen of Death) is description of a specific type of error message from a Windows operating system. A blue screen can be symptomatic of hardware or software issues. A common cause can be a failed or failing storage device. For example, a Windows user will be presented with the stop error "unmountable boot device" while a hard drive is failing or has failed.
BKF - Backup files created by NTBackup Utility and Backup Exec by Veritas.
Byte - Basic measure of memory size. Taking plain text as an example, a kilobyte is likened to a very short story, a megabyte to a small novel, 5 megabytes will hold the complete works of William Shakespeare and a gigabyte of text would be a truck load of paper.
C
Clean-Room - A room with filtered air used for the opening, examination and data recovery from storage devices such as hard drives.
Clone - Cloning a hard drive means you copy the contents of one storage device exactly onto another storage device (usually of same type e.g. S-ATA disk to S-ATA disk). A clone differs from a drive image in that an image is not always bootable and sometimes has to be restored using the software it was created with. Cloning storage devices such as hard drives is used extensively in data recovery processes for non-intrusive recovery. Cloning a storage device, which has suffered data loss to another storage device before the recovery process begins, means the state of the original drive is kept intact.
Cylinder - The sum of all identical tracks of the individual magnetic disks is called a cylinder.
Controller Board - see PCB
CSD - Data File for Collsoft Payroll - a popular payroll application for Irish businesses.
CRC - The Data backup file used by Sage Quickpay payroll software. Used by Irish businesses to calculate wages and statutory deductions.
CTL - Data backup file used by Sage Quickpay. This type of file is used in conjunction with CRC files.
D
DBX - Folders ending with this file extension are Outlook Express email folders and contacts.
Decryption - The process of converting encrypted data back into readable form. This is normally done with an encryption key. In the case of logical damage to a storage device, a decryption program is used before the data recovery process. In the case of physical damage, decryption is performed after physical repair.
Deletion - Under a FAT partition a file gets deleted when its directory entry gets marked with "E5". When the file is saved under an NTFS partition a file gets deleted when its MFT gets marked as unused (but allocation properties are kept intact).
Degausser - An electronic device used to destroy a hard drive's data beyond a recoverable state. The drive is passed through a powerful magnetic field which completely rearranges the polarity of the magnetized particles in the platters rendering any data on the drive useless.
DOC (.DOCX) - The .doc and .docx format is the file extension for the Microsoft Word text processing application. Word documents can become corrupt or deleted for many reasons including accidental deletion, virus attack, template corruption or physical problems with a storage device.
Disk Capacity - The number of bytes stored on an individual hard drive.
Disk Capacity = Number of Heads x Sectors x number of cylinders.
DVR - Digital Video Recorder. A hardware device used by consumers and CCTV systems worldwide for viewing and storing recordings.
DTA - Database file used by the Sage Accounts package. The backup files of the database will often end in .001.
DWG files - The most widely used format for saving AutoCAD drawings. DWG files are interoperable with IntelliCAD and Vectorworks.
DXF (Drawing Exchange Format) - Export file format for AutoCAD. Saving a file in this format makes file readable in other non-AutoCAD applications such as Vectorworks.
E
EEPROM - (Electrically Erasable Program Read Only Memory) A flash memory chip on the PCB of a hard drive which contains unique "DNA" information about the drive. It contains information such as SMART data, systems logs, servo parameters, factory default tables and calibration information that is unique to the drive. Similar information is stored on the drive's system area.
EFS - Encrypting File System - Encryption system for Microsoft platforms. Unlike, Bitlocker (another encryption system from Microsoft) which is primarily designed to encrypt the whole drive - EFS encrypts files based on individual files and is dependent on user accounts associated with it. FAT32 does not support EFS.
EDB (Exchange Database Files) - File extension for Microsoft Exchange emails. Exchange emails can require recovery if there has been an accidental deletion of a mailbox or if "error 550" or "error 528" is showing.
EPUB - An industry standard document format used for e-books. This format has a slight advantage over .PDF on mobile devices as it has better "fit-to-screen" attributes without the user having to over rely on zoom-in functions.
EPS - An Encapsulated PostScript file is a file which stores graphic files as vectors. When a file is stored as vector it can be enlarged any number of times without deterioration of resolution. EPS files can be created by programs such as Adobe Illustrator and Corel Draw.
Encryption - The process of encoding information in an attempt to make it secure from unauthorized access. Common devices which are encrypted by I.T. administrators include laptops, tablet PCs and USB flash drives. Common encryption applications include TrueCrypt, Safeboot, McAfee Endpoint, PGP, Symantec Endpoint, Safeguard, Bitlocker and SafeGuard. Even though a storage device may be wholly encrypted, it is still possible to perform data recovery as long as the user has the encryption key.
External Drive - (e.g. Seagate Free Agent, Western Digital My Book) A hard disk drive which is encased in plastic, metal or rubber and which interfaces with a computer system via a USB, firewire or an E-SATA connection.
F
FAT - A file system used in DOS, Windows and OS/2. It keeps track of where data is stored on a disk. When the term FAT is used without an appended number, it generally refers to FAT16 and FAT32. With FAT32 the largest file is only 4GB. Extended FAT, or exFAT is a new version of FAT for removable media such as USB flash drives. The exFAT file system retains all the advantages of FAT32 but can handle file sizes of up to 32,768 KB.
Filter - For a hard drive to operate perfectly the air inside it must be perfectly clean of contaminants. In order to prevent dust and other fine particles from entering the drive, there is a small filter fitted.
Firmware - Software that is programmed into a hardware device. The firmware of a hard disk drive is usually found on the drive's PCB and system area. The most infamous PCB failures of recent years have been those affecting the Seagate Barracuda 7200.11 series of drives manufactured in 2008. After much user pressure and negative publicity, Seagate finally issued a mealy-mouthed press release and a firmware update. "Some Seagate Barracuda 7200.11 1.5TB hard drives may show uncharacteristic operation when used with Mac and Linux operating systems in multi-drive configurations". (Seagate Press Release 20/11/2008)
Flash Memory - The earliest type of flash memory was developed by Bell Labs in 1967. However, this early type of flash memory was expensive and power hungry.
In 1980, Toshiba filed a patent for a flash memory cells (NOR) which could be written to and erased more quickly. In 1987, a more refined version of NOR flash was developed. NAND flash could be produced more economically in denser arrays and had much greater erase cycles. Today, NAND memory chips are found in everything from digital cameras to USB Flash drives. While being extremely versatile and portable storage devices, USB flash drives or "memory sticks" need data recovery when the "wear-levelling" data stored on the device's controller chip goes corrupt. Another instance where data recovery is needed is when the circuit board of the device gets fully or partially disconnected from the USB connector.
G
G-List (Growth List) - This list of "growing" bad sectors on a hard drive. As a hard drive gets used, the list of it's bad sectors get marked in the G-List. The G-List is usually stored in the System Area of the drive. see also P-List
H
Head Crash - describes the process whereby one or more drive heads "park" themselves on the drive platters. Head crashes are usually caused by sudden power outages or surges and shock damage.
Head (Drive) - The drive head (or drive heads) is the component at the end of the actuator arm which reads data off the drive platters. The distance between the heads and the disk is around 20 to 60 nanometers. In comparison, the thickness of a human hair is around 60,000 nanometers. The drive's heads are the most fragile component inside a modern drive and therefore tend to be the "weakest link" when it comes to drive failure. The most common type of head used in today's hard drives is the tunnelling magnetoresistive head. Unlike inductive heads which detect temporal changes of magnetic flux, magnetoresistive heads sense spatial variations of magnetic fields produced by recorded patterns of magnetisation. Moreover, with the addition of microscopic heater coils to heads (introduced by Seagate in 2005) to improve magnetic transitions, the read and write functions of the head have become even more accurate.
Head Stack Assembly - The component which comprises the actuator, sliders and drive heads.
Hierarchical File System (HFS, HFS+) - The de facto file system used in Apple storage devices. It stores data in a top-to-bottom organisation structure.
I
IDEMA - The International Disk Drive Equipment and Materials Association is the organisation responsible for setting standards and benchmarks for hard disk manufacturers and their suppliers.
iWork - Suite of office applications developed by Apple. The iWork package which includes Pages, Numbers and Keynote is seen by many as Apple's equivalent of the Microsoft Office suite.
IDE (Integrated Data Electronics) - IDE drives use a 40-pin ribbon cable to connect to a motherboard and a 4-pin Molex power adaptor. Transfer rates of between 33 MB/s and 100 MB/s. The IDE drive was overtaken in sales in 2006 by the S-ATA format. IDE drives are sometimes referred to as P-ATA drives.
Initialisation - i) Disk initialisation is the process whereby an MBR is written to a hard drive. In a Windows environment, when a new disk is added to a system the user will be prompted to "initialize the disk". However, this message can also appear unexpectedly when an operating system cannot read or find a valid MBR and can be indicative or logical or physical failure. ii) The term initialisation may also refer to the process of the drive heads successfully deploying on the drive platters upon disk power-up.
J
JBOD - (Just a Bunch of Disks) A JBOD setup denotes a controller-less disk array or disk subsystem. A JBOD does not have a controller and consists only of a case in which multiple hard drives are stored. The hard disk drives operate independently, each having their own address and are used by the server via a single SCSI, IDE or EIDE interface controlled. . JBOD systems have fast access times and are less expensive than RAID systems but offer no redundancy. There are several ways to "JBOD" a RAID system without having to use a RAID controller.
JPEG or JPG - (Joint Photographic Experts Group) A de facto standard for digital photographic images. JPEG was designed to work with images of many different colours and gradations of colour. To minimize the amount of storage space required JPEG compresses information wherever there is a gradual change in colour. This makes JPEG the best format for photographic images. However, JPEG is not a good choice for images that need clear, distinct lines or colour differences. Logos, line art and icons tend to look blurry in JPEG due to its compression, but they look much better in other image formats such as PNG and GIF. As JPEG files are compressed, if a hard drive has logical or physical damage, the MFT information can go corrupt leaving photos unreadable or fragmented but recovery of photos to their original state is still possible.
L
Landing Zone - An area in the inner diameter of a drive platter that is used as a "parking place" for the drive heads. There is usually no data stored in the landing zone area.
Latency - The amount of time it takes for the actuator arm of the drive to position the drive heads over a specific point on the drive platters.
Longitudinal Magnetic Recording - Refers to the position in which data bits are recorded on a drive platter. LMR records across the plane of the platters (lying down). However, there comes a point where the areal density of the platter becomes so "tight", a phenomenon known as the superparamagnetic effect kicks in, as bits lose their previously assigned polarity. This was a problem faced by disk manufacturers in the early 00's, as disk space demands grew exponentially. Therefore, a different technology for recording to high-capacity disks without compromising data stability was developed. This newer technique called Perpendicular Magnetic Recording involves bits being arranged upwards (standing up).
LIF - Low Insertion Force connector socket, pioneered by Intel for use in processors, now employed by hard drive manufacturers such as Toshiba for drive connectors on their 1.8" inch drives.
Linux - A free and stable open source operating system for PCs and Servers developed by Linus Thorwald. File systems for Linux include Ext2, Ext3 and Ext4.
While Linux might not have the same high visibility of Windows, it is used substantially in back-end data processing. Thousands of internet servers and business applications across the world are powered by Linux. (For example, the Frankfurt, Shanghai and Irish stock exchanges all use the Linux-powered Xetra software).
M
MacOS - Developed by the Apple Computer company in 1982. It was the first commercial computer system with a GUI and a mouse. Mac systems primarily use the HFS and HFS+ file system. As Macs use hard drives common to Windows PC's statistically physical failure rates are similar.
MBR (Master Boot Table a.k.a. Partition Table) - The master boot record is the section of drive which contains the "map" or "index" to your drive's data and operating system. If the MBR goes corrupt, the drive will be unreadable to a host system. If a drives MBR cannot be found or repaired a raw data recovery is sometimes needed.
MCU (Micro Controller Unit) - Usually the biggest chip on the PCB of a hard drive.
During read operations it converts analogue signals into digital signals which are then sent via the MCU's I/O port to the drives interface (S-ATA etc.) During write operations it converts digital signals into analogue.
MDB files - The file extension for Microsoft Access database files.
MFT - Information about stored files can be found in the Master File Table. The MFT tells us the name of the file, date, time, size and clusters occupied by the file.
Mirroring - see RAID 1
.MP3 - The most popular audio format is MP3. An MP3 file is a compressed version of the original audio recording, so its sound is not as good as a professionally mastered CD, but this format's popularity worldwide indicates that quality is good enough for most people. Professionals who need to work with original audio like composers and those who work in media-related industries often prefer to work with .WAV files and .AIFF files.
MPEG - (Moving Pictures Expert Group) Mpeg is a final product video format and enables quality video using smaller file sizes than AVI.
MSF (Mail Summary File) - Mail store folder as used by the Mozilla Thunderbird email application.
MySQL - Database which used SQL. Data recovery of a MySQL database is often needed when .myi .myd and .frm files go corrupt due to logical damage or physical damage to a storage device.
N
NAS (Network Attached Storage) - A storage device used as a file-server or backup device which uses RAID or JBOD. NAS is often used as a file server or back-up device in an networked environment. Easier to maintain than a dedicated file server such as Windows Server 2008 but still offering better data protection than a standalone external hard drive. Common NAS devices include those from Drobo, Iomega, G-Tech, Synology and LaCie.
NSF (Notes Storage Facility) - The file extension for Lotus Notes.
NTFS - The native file system for Windows.
NVRAM -Chip holding drive calibration data found on the PCB of a hard drive. Usually found in addition to the EEPROM chip. While the EEPROM chip will usually hold drive specific information an NVRAM chip holds additional drive calibration data. Commonly found on Hitachi Deskstar and Travelstar drives.
P
Pages - The word processor application developed by Apple. This application was previously known as Apple Works. It uses the .pages file extension.
Parity - A computation for RAID using XOR.
PATA - see IDE
PCB - (Printed Circuit Board a.k.a Logic Board). Electronic circuit board which is attached to the underside of a hard drive. This board contains a flash memory chip which holds firmware specific to the drive. See also EEPROM.
PDF - Portable Document Format - An "almost" universal file format for documents. A PDF document's greatest advantage is its share-ability: any Mac, Windows or Linux PC can open and read PDF files without having to buy special software. The only drawback of PDF files is that they're read-only, not editable by recipients. (There are ways of editing PDF files, but they require special software).
P-List - Usually held in the system area of the drive, the permanent defect list (P-List for short) contains all the permanent bad sectors which have been discovered by the manufacturer during the quality control stage of manufacture.
PPT, PPTX - The file extension for Microsoft's presentation application PowerPoint.
PSD files - The native format of Photoshop image files.
PST file - The main storage file as used by the Microsoft Outlook email client. The .pst usually stores all inbox, sent items and contacts folders. The .pst file can become inaccessible due to corruption of the file itself or due to logical or physical damage of the storage device.
Perpendicular Magnetic Recording - A new way of storing data on hard disks that surmounts some of the limitations of Longitudinal Magnetic Recording. By stacking data bits on end (standing up), instead of across a plane, PMR allows greater data densities and storage capacities to be achieved. The first commercial disk using PMR technology was released by Toshiba in 2005.
Preamp (Preamplifier) - The preamplifier controls which drive head gets deployed to read a given sector or track. This component also has the function of amplifying the high frequency signal detected by the heads. If not amplified, the signals would be attenuated to such a degree they would be undetectable to the drive controller.
Portable Network Graphics (PNG, pronounced Ping). PNG files are primarily used for graphics, templates, layouts and transparent images.
Q
QBW files - Data files created by the widely used accounts package QuickBooks. This accounts application is used by small and medium sized businesses and has an estimated 25 million users worldwide.
R
RAID (Redundant Array of Independent Disks) - A system that uses multiple hard drives to store data. The basic principle of RAID (Redundant Array of Independent Disks) is to combine multiple inexpensive disk drives into an array to obtain capacity, performance and reliability that is superior to a single large drive. The array of drives appears to the host computer as a single logical drive. While RAID offers a greater level of redundancy than a single volume drive, RAID data recovery is needed when one (or more) disks in the array fail or the parity of the array gets corrupted.
RAID 0 - RAID 0 is the lowest form of RAID and seen by some as not a valid form of RAID at all because there is no redundancy. Data is striped across two drives and the user sees as one volume. The main rationale for implementing a RAID 0 setup is enhanced performance. Because there are two drives, data can be written or read at nearly twice the speed of a single drive.
RAID 1 -With a RAID 1 configuration the exact same data on the first disk is written to the second disk, also known as mirroring.
RAID 5 - A type of RAID where data and parity blocks are written to multiple disks (three or more) simultaneously. Parity blocks are written across all drives in the array.
RAW - A type of data recovery where the disk has severely corrupted partition or master file table corruption. Only used as a last resort as the files recovered will usually lose their file names and original storage paths.
Rotational Speed - The average rotational of a standard S-ATA hard drive is 7200 rpm. The fastest drives such as Western Digital's Raptor range of drives have specified seek times of 10000 rpm. Rotational speed is by no means a good indication to overall disk performance. It is not uncommon for instance, that a 5400rpm to beat a 7200rpm drive in independent performance tests. Overall, disk performance will always be determined by other factors such as transfer rate, caching and interleaving.
RPT - Crystal Reports File which can be opened with SAP Crystal Reports or Microsoft Visual Studio.
S
S-ATA - Successor to IDE drives, commercially introduced in 2003. Transfer speeds of 150 MB/s for S-SATA I and 300 MB/s for S-ATA II.
SECTOR - The small unit of storage that is addressable on a hard drive. Traditionally, a drive's small sector has been 512 bytes. By 2011, it is expected that most drive manufacturers will have made the transition from 512 bytes to 4 kilobytes as the smallest addressable unit. See also Bad Sector
SCSI (Small Computer Systems Interface) - Most PCs use an ATA bus instead of SCSI for connecting internal hard disks. SCSI is more often seen in servers, as it tends to be faster and more reliable (though more expensive). A SCSI controller can handle 7 devices whereas an ATA can handle maximum 2.
System Area - Area reserved on the inside or outside tracks of the drive platters for essential drive related information. The system area will contain information such as the P-List, G-List and drive head calibration information. In the case of PCB failure successful data recovery by means of a PCB board swap with an identical drive alone is rarely successful as other firmware must be extracted from the system area.
Solid State Drive - A hard drive which uses flash memory instead of disk platters and a head disk assembly. First commercial SSD drives appeared in 2007. Data recovery from solid state drives is often still needed however. Corruption to the controller chips, system area information and physical damage to the PCB board is still possible.
Shock Proof Drives - (e.g. LaCie Rugged All Terrain) External hard drives which can theoretically withstand shocks or impacts. In reality, most are just standard hard drives using rubber mounts and casing. If a user really wants an external drive with better than average probability of surviving a shock or fall unscathed, it would be best to acquire a 2.5"inch drive with a 5400 rpm or use a solid state drive. But ultimately, the best form of shock protection is prevention. Endeavour to always have your data backed up on at least two or more devices. Even the safest Volvo can hit a wall.
Spindle Motor - Device which spins the platters. The average speed of a spindle motor is 7200rpm.
SMART - Self Monitoring Analysis and Reporting Tool - Self-diagnosis by hard drives to prevent data loss. SMART measures read/write failures and in certain cases notifies users of any impending data loss. The technology is an industry standard developed by IBM and Compaq. In a 2007 study of hard drive failure conducted by Google in one of their data centres, 56 per cent of SMART enabled drives that failed did not report any errors before failure. SMART should only ever be seen as an extra safeguard (albeit inconsistent) in ascertaining hard drive health and should never be a substitute for complete hard drive diagnostics.
SQL (Structured Query Language) - Relational database language developed by IBM in the early 1970's. SQL is widely used in databases such as MySQL and Oracle.
Stepping - The process whereby the drive head(s) is moved by the actuator arm from one disk track to another.
Striping - The size of "data chunks" written across a RAID array. In terms of RAID data recovery, ascertaining the exact striping size used in the array is a crucial step in the recovery process.
T
Tablet computer - A portable personal computer with a touch screen interface. Typically, a tablet is larger than a smart phone but smaller than a laptop. Tablet PCs usually use solid state drives or 1.8" (ZIF or LIF) or 2.5" (IDE or S-ATA) drives for storage.
TB2 - File extension for TasBooks 2 accounts package. Tasbooks is an accounts package used primarily by Irish small and medium sized businesses.
TB3 - File extension for TasBooks 3 accounts package.
Time Capsule - Apple branded external storage device. These can be synced with Time Machine software for data backup. Many of these devices now come with an ethernet port and can be used as a NAS. Inside the glossy casing, a Hitachi Deskstar drive can usually be found.
Transfer Rate - There are two different transfer rates. The internal transfer rate is the speed of data transfer from disk platter to the head. While the external transfer rate is the speed between the hard drive controller and the host computer.
TVS - A Transient Voltage Suppression diode protects a hard drive from power surges. There are usually two diodes on every PCB, one for 5V protection and the other for 12V protection. Most TVS's are able to protect a drive from mild power surges. For a more robust power surge protection solution, a UPS is recommended.
Trojan - A malicious program disguised as something benign. Trojans are often downloaded along with another program or software package. Once installed on a system, they can cause data loss by executing "delete" commands surreptitiously.
TrueFFS - True Flash File System is a low level file system for flash and solid state drives. It emulates error correction and bad block remapping which a traditional hard drive employs.
U
UFS - File system used by UNIX systems. Less prone to file fragmentation than NTFS.
UPS - (Uninterruptible Power Supply) - A device which acts as an intermediary between a mains power supply and a server (or PC). A UPS ensures consistent power during a power surge or outage. By preventing the interruption of power, drive defects such as head crashes can be prevented. Moreover, it often gives computer users enough time to save open files and have an orderly shut-down of PCs to prevent damage to file data structures.
V
VCM Controller (Voice Coil Motor) - The VCM controller controls the spindle motor and head rotation of the drive. The VCM chip is found on the PCB.
VDMK - File extension used by VMware software.
Virus - Malicious code written with the intention to damage the user's computer. A virus can logically damage disk clusters, batch files and macros (such as Microsoft Word and Excel).
VWX - File extension used by for saving 2-D and 3-D graphic files using Vectorworks CAD.
W
Wear Levelling - A technique to evenly distribute read/write functions throughout the cells of a memory chip. Each cell in the NAND chip has maximum number of read/write functions during its lifetime, beyond which the cell becomes defunct. Wear levelling technology strives to make memory cells from becoming over-used by evenly distributing the read/write functions. With equal use of all cells within a chip their life cycle is maximised.
Worm - Malicious code designed to self-replicate. One of the first and most published cases of a worm causing the deletion of documents, music and multimedia files was the Lovebug worm released in May 2000.
X
Xcopy - A Windows command for copying directories or multiple files from one drive to another. This command is used by I.T. administrators to extract data from a drive quickly if they believe failure is imminent.
Xor (eXclusive OR) - Data recovery from RAID relies on the Boolean logic of Xor to reconstruct a corrupted array. Xor analyses a series of 0's and 1's resulting either "true" or "false". Even numbers are designated "true" while odd numbers are "false". By using this logic, the controller can fill in the blanks - just like in algebra. We know that 2+3=5. If we see an equation like 2+x=5. We know that "x" is meant to be a 3. Xor uses the same principle for maintaining RAID integrity.
XLS - The file extension for Microsoft's Excel spreadsheet application. From Microsoft Office 2007 onwards, Excel uses the file extension XLSX.
XFS - Journaling file system for UNIX. Commonly used formatting partition table in NAS devices because of it's suitability in handling large files and offers smooth data transfer.
Z
Zettabyte - A zettabyte is equal to 1 billion terabytes. A 2010 survey by IDC predicts the amount of information stored worldwide in digital format in 2020 will be 35 zettabytes or 44 times more data than stored in 2009.
Zone Bit Recording - The classic assignment of space on a hard drive based on the geometric structure of its sectors, cylinders and write heads. The Zone Bit Recording (ZBR) departs from this basic structure, because the number of sectors on the hard disk is different. At the outer edge of the disk, there more sectors per track than in the middle. Therefore, disk storage evenly distributed in zones that have a different number of sectors. At a constant rotational speed of the platters, the data transfer rate depends on the track. In the middle, it is lowest and at the outer edge the highest.
Zif (Zero Insertion Force) Connector - Type of connector used on 1.8" hard drives for netbooks, laptops, tablet PCs and digital camcorders. Unlike a LIF interface, ZIF has a tightening clasp.
Zip Disk - The ZIP disk developed by Iomega and having the standard format of 98 x 97 x 4 mm, requires its own ZIP drive and has a storage capacity of 100 megabytes (MB), 250 MB and 750 MB. The access times ranges from 20 ms to 40 ms, the data transfer rate of 750 kb / s, 1.5 MB / s and 7 MB / s with USB 2.0. The Zip drive on a system can be interfaced by IDE , SCSI , USB and via a parallel port.
Zip Files - Files which have been compressed to a size smaller than their original size. Zipped files take up less space on a hard drive and are quicker to download from the internet or send via email.
