Hard Disks (Hard Drives)

A hard disk is a magnetic disk made of metal and covered with a magnetic recording surface. Hard disks come in removable and fixed varieties that hold from several hundreds of megabytes to several gigabytes. They are tightly sealed to prevent any foreign matter from getting inside, which causes head crash.

Interface Type

• IDE(Integrated Drive Electronics): An IDE interface has a disk drive that contains its own controller electronics. The IDE interface is also called an AT and XT interface. IDE-ready motherboards have a 40-pin socket that connects directly to an IDE drive eliminating the use of an expansion slot.

• SCSI (Small Computer System Interface): SCSI is pronounced scuzzy. This is an 8-bit-bus peripheral interface for up to seven peripherals. The SCSI bus allows any two devices to communicate at one time (host to peripheral, peripheral to peripheral). SCSI provides high-speed (4MB/sec.), parallel data transfer and multiple peripheral connections while taking only one expansion slot.

• Access time: This is an average time taken to complete the transfer of data after the request instruction has been enacted. Today's fast hard drives have access times under 10 milliseconds (ms). Access time is made up of the following four times.

• Seek Time: This is the time taken to move an access arm to a certain track on a disk after the computer requests data. Most of the access time is made up of seek time.

• Head Switching Time: The time taken for changing from one read/write head to another to read from or write on another part on a disk.

• Search Time: It is also called rotational delay time. This is a time required for the read/write head to locate particular position on a track.

• Data Transfer Time: This is the time for data to be transferred from the disk to primary storage or vice versa.
  

• Internal Hard Disk: Internal hard disk is made up of several metallic platters, a motor, an access arm and read-write heads sealed inside a container.
  An internal hard disk is looked like a part of a system unit inside a computer cabinet. There are two sizes of drives (5.25" and 3.5" in a diameter). 3.5" hard disks are faster because the access arm travels shorter distances across the diameter of the disk.
  Internal hard disks have advantages over flexible disks. They are high capacity and speed. The disadvantage of internal hard disks or hardcards is that they have only a fixed amount of storage and cannot be easily removed.

• External Hard-Disk Drives: This is a drive that is not built into the system cabinet of microcomputers. External hard disk drives are treated as peripherals. Using external hard disk drives, we can expand the hard disk capacity when all available drive bays are occupied.

• Hard Disk Cartridges: A cartridge is a removable storage module, so a hard disk cartridge contains disks in the module. They can be removed from a dock easily and can give fast access to large data. An internal or external dock is available.
  In internal hard disks and external hard disk drives, the storage capacity is fixed, but in the hard disk cartridge, the capacity limitation of storage does not exist. That is, a user may add more cartridges any time.

• Removable Drives with Cartridges: Today's advanced technology allows a new form of removable storage. The technology combines the function of the hard disk drive and the convenience of the hard disk cartridge. This form consists of a removable drive and several cartridges.
  The removable drive looks like an external floppy disk drive. And the cartridge resembles a floppy disk and allows users to add 100 MB or 1 GB at a time. It allows an SCSI connection as well as a parallel port connection. Its low cost is a big advantage.

Fast SCSI: Fast SCSI provides 10MB data transfer through an 8-bit-bus.

Wide SCSI: Wide SCSI provides up to 40MB transfer speed with a secondary "B" cable for 16-bit and 32-bit data.

Tightly Sealed Container: A hard disk is enclosed in a tightly sealed container. The reason is to prevent any foreign material from getting inside. The read-write head rides on a cushion of air about 0.000001- inch thick. It is so thin that a smoke particle, fingerprint, or human hair could cause a head crash.

Head Crash: This is a physical destruction of a hard disk. Misalignment or contamination with dust can cause the read/write head to collide with the disk's recording surface. The head crash destroys all or part of data on the disk.

Hardcard: A hardcard is a circuit board with a disk that plugs into an expansion slot in the computer. The card is only 1-inch-thick. This can be an alternative to the external hard-disk drives.