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Hard Drives

Hard Drives A hard drive (HDD) is usually the main storage device in a PC, though Solid State Drives are gaining in popularity. Hard drives contain magnetic coated platters where data is stored that rotate at high RPMs. The five main interfaces used with hard drives are EIDE, SCSI, USB, FireWire and Fibre Channel.

Hard Drive Interfaces

  • IDE

    The majorty of hard drives are EIDE hard drives. EIDE is used with other devices as well, such as CD and DVD drives, zip drives and tape drives. EIDE Continued...

  • SCSI

    SCSI is a parallel interface standard for attaching peripheral devices to a PC. SCSI provides transfer rates up to 80 MB per second and depending on the standard the SCSI bus can contain up to 15 devices. SCSI Continued...

  • USB

    USB is a popular interface for external hard drives. USB 1.0, the first USB standard, was not fast enough to be used with hard drives so it was not used until later when USB 1.1 and 2.0 were introduced. USB Continued...

  • FireWire

    FireWire, also know as i.Link by Sony, uses serial transmission for data. A FireWire hard drive connects through a PC using a 1394 external port, internal 1394 expansion card or 1394 port built directly into the motherboard. FireWire Continued...

  • Fibre Channel

    Fibre channel is a type of SCSI hard drive technology used in high-end systems with multiple hard drives installed. Fibre Channel Continued...

How A Hard Drive Functions

Hard drives use one or more round magnetic disks called platters, read/write heads controlled by an actuator, and a spindle on which the platters rotate.

Each side of a platter, known as a head, is used to store data. Each head of a platter is divided into tracks and sectors (each sector is 512 bytes large). Their is a track on each head and all the tracks together form the cylinder. Data is written to hard drive platters beginning at the outer track, moving inward as sectors are filled.

Tracks and Sectors

Current hard drives use zone bit recording in which the tracks near the center have the smallest number of sectors per track. The number of sectors increases as the size of the tracks increase. This means each track has the optimum amount of sectors for the size of the track. This differs from devices such as floppy drives in that floppies use the same number of sectors for every track and hard drives with zone bit recording can have more sectors as the size of the tracks increases.

Hard Drive File Systems

With the current line of Windows operating systems, you have a choice of three file systems. FAT16, FAT32 and NTFS (New Technology File System). FAT is short for File Allocation Table.

  • FAT16

    All versions of windows support the FAT16 file system. FAT16 uses 16 bits for each cluster entry. The smallest cluster size of four sectors, which is 2,048 bytes ( 4 sectors * 512 bytes per sector).

  • FAT32

    Microsoft begain offering a FAT that held 32 bits per entry with Windows 95 OSR2. FAT32 is most efficient with drives up to 16 GB. Windows 2000 and XP only support drives up to 32 GB with FAT32. Drives larger than 16 GB require a more powerful file system. That file system is NTFS.

  • NTFS

    The NTFS file system offers better performance and is more secure than FAT. NTFS uses a database called MFT (Master File Table) to hold records about files and their locations. NTFS is only supported in the NT/2000/XP versions of Windows and is designed for the larger capacity hard drives currently being used. Their are presently five versions of NTFS: v1.0, v1.1, v1.2, v3.0 and v3.1. They are also know as v4.0, v5.0 and v5.1, after the version of Windows they shipped with.

RAID (Redundant Array of Independent/Inexpensive Disks)

RAID uses multiple hard drives that function as one large drive and provide improved performance, larger capacity and fault tolerance. RAID can be setup using either hardware or software. In order to use hardware RAID, your hard drive controller or motherboard must support RAID.

RAID 0

RAID 0, also known as a striped volume, can use space from two or more hard drives to split data evenly across the drives to increase disk space. RAID 0 can be used to increase performance, but offers no fault tolerance.

RAID 1

RAID 1, also known as a mirrored volume, duplicates data across two or more drives. RAID 1 offers fault tolerance. Each drive has its own volume. The combined volumes are known as mirrors. If one of the drive fails, the other can take over and no data will be lost. Drive duplexing used two or more controllers for each drive and provides better fault tolerance than mirroring.

RAID 2

In RAID 2 data is striped at the bit level. Very high transfer rates are possible with RAID 2, however, it is no longer used today.

RAID 3

In RAID 3 data is striped at the byte level. In RAID 3, striping is configured so that each individual disk I/O operation is spread across all the drives in an array. RAID 3 is best for single user systems with sequential disk I/O to provide increased performance and fault tolerance.

RAID 4

RAID 4 is similar to RAID 3, except that it uses blocks instead of bytes for striping.

RAID 5

RAID 5 is striped across three or more drives. RAID 5 uses parity checking. This means if one drive fails, the other drives can recreate the data that was stored on the failed hard drive. RAID 5 increases capacity, provides fault tolerance and improves performance. This makes it one of the most popular levels of RAID.

Operating System Hard Drive Size Restrictions

While hard drive capacity is always increasing, operating systems still have limitations to the size of volumes (logical drive) which they can support.

  • DOS and Windows 9x

    Volumes no larger than 2.1 GB in FAT16. Windows 9x does not support hard drives larger than 137 GB in FAT 32.
  • Windows NT/2000/XP

    Limit of 4 GB per volume in FAT16 and 32 GB/volume in FAT32.
  • Windows 2000

    Supports hard drives larger than 137 GB as long as Service Pack 3 or higher has been installed.
  • Windows XP

    Supports hard drives larger than 137 GB as long as Service Pack 1 or higher has been installed.

The capacity or size of a hard drive is not the same thing as a volume on a hard drive. A volume is a portion of a drive that is seen by the OS as if it were a seperate physical drive. Each volume is assigned a drive letter. With the FAT32 and NTFS file systems, a volume can be the size of an entire hard drive.

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