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About Drives


Hard Disk:
The primary computer storage medium, which is made of one or more aluminum or glass platters, coated with a ferromagnetic material. Most hard disks are fixed disks, which are permanently sealed in the drive. Removable cartridge disks such as Iomega's Jaz disks enable the disk to be removed from the computer and used as backup or transferred to another machine with the same drive.

Most desktop hard disks are either IDE (also known as EIDE or ATAPI) or SCSI. The advantage of IDE is their lower cost. The advantage of SCSI is that up to seven or more devices can be attached to the same controller board. SCSI drives are typically used in high-end servers, because SCSI is available as a fault tolerant disk subsystem (RAID systems), while IDE drives are found in most desktop and laptop machines. Increasingly, IDE drives are available in RAID configurations (see RAID).

Hard disks provide fast retrieval because they rotate constantly at high speed, from 5,000 to 15,000 rpm. In laptops, they can be turned off when idle to preserve battery life.

Back in the 1950s, the very first hard disks held just a few hundred thousand bytes and used platters 12" in diameter. In the 1980s, the first personal computer hard disks started at 5MB (see ST506). Today's hard disks start around 20 gigabytes and generally use 3.5" platters for desktop computers and 2.5" platters for notebooks.

Hard disks are usually low-level formatted from the factory, which records the original sector identification on them. See floppy disk, magnetic disk and format program.

Hard Drive:
The primary computer storage device, which spins, reads and writes one or more fixed disk platters. In practice, the terms "hard drive" and "hard disk" are used synonymously. Hard drives are the storage medium in desktop and laptop computers as well as all servers and mainframes throughout the world. They are also used in printers for storing fonts and print jobs as well as MP3 players and a myriad of other portable and stationary computer-based devices. Although removable disks encased in cartridges use the same "hard" disk media and a similar drive technology, they are mostly called "removable drives" rather than hard drives.

The term "hard" differentiates high-capacity rigid disks made of aluminum or glass from low-capacity floppy disks made of plastic.

Megabyte - MB:
One million bytes, or more precisely 1,048,576 bytes.

Gigabyte - GB:
One billion bytes.

The connection and interaction between hardware, software and the user.

Hardware interfaces are the plugs, sockets, wires and the electrical pulses traveling through them in a particular pattern. Also included are electrical timing considerations. Examples are RS-232 transmission, the Ethernet and Token Ring network topologies and the IDE, ESDI, SCSI, ISA, EISA and Micro Channel interfaces.

Software, or programming, interfaces are the languages, codes and messages programs use to communicate with each other and to the hardware. Examples are the applications that run under the Mac, DOS and Windows operating systems as well as the SMTP e-mail and LU 6.2 communications protocols.

User interfaces are the keyboards, mice, commands and menus used for communication between you and the computer. Examples are the command lines in DOS and UNIX, and the Mac, Windows and Motif graphical interfaces.

Interfacing is a major part of what engineers, programmers and consultants do. Users "talk to" the software. The software "talks to" the hardware and other software. Hardware "talks to" other hardware. All this is interfacing. It has to be designed, developed, tested and redesigned; and with each incarnation, a new specification is born that may become yet one more de facto or regulated standard.

Transfer Rate:
Also called "data rate," it is the transmission speed of a communications or computer channel. Transfer rates are measured in bits or bytes per second.

Pronounced "cash." A cache is used to speed up data transfer and may be either temporary or permanent. Memory and disk caches are in every computer to speed up instruction execution and data retrieval. These temporary caches serve as staging areas, and their contents can be changed in seconds or milliseconds.

Browser caches and Internet caches hold popular Web pages long periods of time and even for the duration, because caching servers constantly update the page with the latest version from the Internet (see Web cache and browser cache). In these cases, the cache database is actually a folder on the disk.

Disk Caches:
A disk cache is a section of main memory or memory on the disk controller board that bridges the disk and the CPU. When the disk is read, a larger block of data is copied into the cache than is immediately required. If subsequent reads find the data already stored in the cache, there is no need to retrieve it from the disk, which is slower to access.

If the cache is used for writing, data is queued up at high speed and then written to disk during idle machine cycles by the caching program. If the cache is built into the hardware, the disk controller figures out when to do it.

Access Time:
Disk access time is an average of the time between initiating a request and obtaining the first data character. It includes the command processing, the average seek time (moving the read/write head to the required track) and the average latency (rotation of disk to the required sector). This specification must be given as an average, because seek times and latency can vary depending on the current position of the head and platter.

Fast hard disks have access times of 10 milliseconds (ms) or less. This is a common speed measurement, but overall disk performance is significantly influenced by channel speed (transfer rate), interleaving and caching.

(Revolutions Per Minute) The measurement of the rotational speed of a disk drive. Floppy disks rotate at 300 rpm, while hard disks rotate from 2,400 to 3,600 rpm and more.

(Basic Input Output System) An essential set of routines in a PC, which is stored on a chip and provides an interface between the operating system and the hardware. The BIOS supports all peripheral technologies and internal services such as the realtime clock (time and date).

On startup, the BIOS tests the system and prepares the computer for operation by querying its own small CMOS memory bank for drive and other configuration settings. It searches for other BIOS's on the plug-in boards and sets up pointers (interrupt vectors) in memory to access those routines. It then loads the operating system and passes control to it. The BIOS accepts requests from the drivers as well as the application programs.

BIOSs must periodically be updated to keep pace with new peripheral technologies. If the BIOS is stored on a ROM chip (ROM BIOS), it must be replaced. Newer BIOSs are stored on a flash memory chip that can be upgraded via software.

(2) (Integrated Development Environment) A set of programs run from a single user interface. For example, programming languages often include a text editor, compiler and debugger, which are all activated and function from a common menu.

(1) (Integrated Drive Electronics) A type of hardware interface widely used to connect hard disks, CD-ROMs and tape drives to a PC. IDE is very popular because it is an economical way to connect peripherals. Starting out with 40MB capacities years ago, 20GB IDE hard disks have become entry level, costing less than half a cent per megabyte.

With IDE, the controller electronics are built into the drive itself, requiring a simple circuit in the PC for connection. IDE drives were attached to earlier PCs using an IDE host adapter card. Today, two Enhanced IDE (EIDE) sockets are built onto the motherboard, and each socket connects to two devices via a 40-pin ribbon cable. Starting with ATA-66 drives, the cable uses 80 wires and 39 pins. It plugs into the same socket with one pin removed.

The IDE interface is officially known as the ATA (AT Attachment) specification. ATAPI (ATA Packet Interface) defines the IDE standard for CD-ROMs and tape drives. ATA-2 (Fast ATA) defined the faster transfer rates used in Enhanced IDE. ATA-3 added interface improvements, including the ability to report potential problems. Starting with ATA-4, either the word "Ultra" or the transfer rate was added to the name in various combinations. For example, at 33 Mbytes/sec, terms such as Ultra ATA, Ultra DMA, UDMA, ATA-33, DMA-33, Ultra ATA-33 and Ultra DMA-33 have all been used. Following are the transfer rates for the various ATA modes. See Cable Select.

IDE Drive Type PIO Mode Transfer Rate MB/Sec DMA Mode Transfer MB/Sec
ATA 0 3.3 0 4.2
ATA 1 5.2 - -
ATA 2 8.3 - -
ATA-2, 3 3 11.1 1 13.3
ATA-2, 3 4 16.6 2 16.6
ATA-4 (ATA-33) - - 2 33.3
ATA-5 - - 0 16.6
ATA-5 - - 1 25.0
ATA-5 (ATA-33) - - 2 33.3
ATA-5 - - 3 44.4
ATA-5 (ATA-66) - - 4 66.6
ATA-6 (ATA-100) - - 5 100.0

(Small Computer System Interface) Pronounced "scuzzy." SCSI is a hardware interface that allows for the connection of up to 15 peripheral devices to a single board called a "SCSI host adapter" that plugs into the motherboard, typically using a PCI slot. SCSI peripherals are daisy chained together. They all have a second port used to connect the next device in line. SCSI host adapters are also available with two controllers that support up to 30 peripherals.

Introduced in 1986 and originally developed by Shugart Associates (see SASI), SCSI is widely used from desktop PCs to mainframes, although most desktop PCs come with IDE drives. The advantage of SCSI in a desktop PC is that a scanner and several other drives (CD-Rs, DVD-RAM, Zip drives, etc.) as well as hard drives can be added to one SCSI cable chain. However, this has become less important as alternate interfaces such as USB and FireWire have become popular.

Until the late 1990s, SCSI hard disks were the only ones used in RAID configurations which provide improved performance and/or fault tolerance. Since the advent of IDE RAID controllers, SCSI and IDE have become more equalized, although SCSI continues to be the drive interface of choice in the server market.

Windows 95/98/NT/2000 and the Macintosh provide internal support for SCSI, but Windows 3.1 and DOS did not. Installing SCSI in a Win 3.1 or DOS machine required adding the appropriate SCSI driver.

A SCSI Chain:
The advantage of SCSI is that several peripherals can be daisy chained to one host adapter, using only one slot in the bus.

Because internal support for SCSI was not provided by DOS and Windows 3.x, there was no benchmark for a standard implementation. As a result, hooking up two SCSI devices often meant plugging in two host adapters, negating SCSI's advantage of connecting multiple peripherals.

ASPI and CAM were created to resolve these differences and provide common interfaces between the drivers and the host adapters. Almost all SCSI products are ASPI and CAM compliant. Windows 95 and higher does support popular SCSI host adapters directly. It also supports the ASPI and CAM standards so that older applications and drivers will run even if Windows does not support that peripheral with a native driver.

SCSI Is Like a LAN:
SCSI is a bus structure itself and functions like a mini-LAN connecting eight or 16 devices. The host adapter counts as one device, thus up to seven or 15 peripherals can be attached depending on the SCSI type. SCSI allows any two devices to communicate at one time (host to peripheral, peripheral to peripheral).

SCSI Specification

Type Bus Width (bits) Max Dev Transfer Rate
Bus Lengths, Meters for Device Types: Pins
Single- ended LVD HVD
SCSI-1 8 8 5 6 12* 25 25
SCSI-2 8 8 5 6 12* 25 50
Fast SCSI 8 8 10 3 12* 25 50
Wide SCSI, aka
Fast Wide SCSI
8 16 20 3 12* 25 68
Ultra SCSI 8 8 20 3 - - 50
Wide Ultra SCSI 16 16 40 - 12* 25 68
Wide Ultra SCSI 16 16 40 1.5 - - 68
Wide Ultra SCSI 16 16 40 3 - - 68
Ultra2 SCSI 8 8 40 - 12 25 50
Wide Ultra2 SCSI 16 16 80 - 12 25 68
Ultra3 SCSI, aka
Ultra160 SCSI
16 16 160 - 12 - 68
Ultra4 SCSI, aka
Ultra320 SCSI
16 16 320 - 12 - 68

12* - LVD was not part of these specs; however, if all devices are LVD, 12 meters applies. If any device is single ended, then length in SE column applies.

Information for this chart was obtained from the SCSI Trade Association (STA), San Francisco, CA (

Version Compatibility:
The different SCSI types provide backward and forward compatibility. If a new SCSI host adapter is used with an older SCSI drive, the drive will run at its maximum speed. If an older SCSI host adapter is used with a newer drive, the drive will run at the host adapter's maximum speed.

SCSI and IDE Drives:
You can install SCSI hard disk drives in a PC that already contains one or two IDE disk drives. The IDE drive will still be the boot drive, and the SCSI drives will provide additional storage. Follow the instructions in your SCSI host adapter manual carefully to make the correct settings. Some SCSI host adapters provide floppy disk control, which can be disabled.

IDs and Termination:
SCSI devices are daisy chained together. External devices have two ports, one for the incoming cable and another for the outgoing cable to the next device. An internal device has a single port that attaches to a ribbon cable with multiple connectors. Each device must be set to a unique ID number, which is normally done by flipping rotary switches on external devices or by setting jumpers on internal ones. The SCSI ID determines the device priority, which starts at 7 and goes to 0 and then from 15 to 8. The host adapter defaults to the highest priority, which is 7.

A subset of Plug and Play, called "SCSI Configured Automatically" (SCAM), allows IDs to be set by software rather than manually. Both the host adapter and peripheral must support this.

The device at the end of a SCSI chain must be terminated by either setting a switch or plugging a resistor module into the open port. Usually, host adapters default to terminated. If devices are connected both internally and externally, the host adapter termination must be removed and termination must be applied to the ends of both chains.

Parallel to SCSI:
There are adapters that allow SCSI peripherals to be connected via the parallel port. Although the parallel port's transfer rate is considerably less than the SCSI host adapter, it does provide a means to hook up SCSI devices to laptops without PC Card slots or desktop machines without available bus slots.

Each SCSI device can be further broken up into eight logical units, identified by logical unit numbers (LUNs) 0 to 7. Although most SCSI disks contain only one disk inside and are addressed as LUN 0, CD-ROM and optical disk jukeboxes contain multiple units. Each disk in these devices can be addressed independently via LUN numbers; for example, a four-disk jukebox could be assigned LUN 0 to 3.

Single Ended, Differential and Low Voltage Differential:
There are three types of SCSI signaling. Single-ended SCSI allows devices to be attached to a total cable length of 6 or 3 meters for Fast and Ultra SCSI. Single-ended SCSI is not defined for Ultra2 SCSI and higher.

Differential SCSI, or High Voltage Differential SCSI (HVD), is used when devices are spread across a room, because the total cable length is increased to 25 meters. Differential devices cost more than single-ended ones.

Ultra2 SCSI introduced Low Voltage Differential signaling (LVD or LVDS) that supports cable lengths up to 12 meters. Single-ended SCSI uses a data line and ground. Both HVD SCSI and LVD SCSI use data low and data high lines to increase transmission distance. However, LVD requires less power and is less costly, because the transceivers are built into the controller chips.

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