Friday, December 5, 2008

COMPUTERS - Hard Drives

Technically, what are hard drives in the computer world?

The generic category is Storage Device, but this applies to more than hard drives. Example, a USB Flash Drive is also a Storage Device.


A hard disk drive (HDD), commonly referred to as a hard drive, hard disk, or fixed disk drive, is a non-volatile storage device which stores digitally encoded data on rapidly rotating platters with magnetic surfaces. Strictly speaking, "drive" refers to a device distinct from its medium, such as a tape drive and its tape, or a floppy disk drive and its floppy disk. Early HDDs had removable media; however, an HDD today is typically a sealed unit (except for a filtered vent hole to equalize air pressure) with fixed media.

Read the full Wikipedia article for more info


First, HOW HDDs physically do what they do is a hardware issue. Your Operator System (OS) only deals with how file-data is written to the HDD and the OS has its own way of tracking the files.

Your OS gets the physical address references from your HDD(s), essentially:
  • The physical address of each drive from BIOS (HDD0, HDD1, etc.)

  • The HDD physical addressing scheme is called LBA (Logical Block Addressing)
  • For each HDD, the Master Boot Record (MBR) located on Platter 0, Track 0, Cylinder 0 (aka LBA0)

Among other things, the MBR contains the Primary Partition Table (PPT).

IMPORTANT NOTE: The MBR is written to a special DOS FAT partition. What makes is special is that it MAY be a non-standard DOS size. The minimum DOS FAT size is 512kb (this is what Floppies are formatted in) and multiples (514/1024/1536/etc), but this special MBR sector MAY be less than 512kb. This sector could also include (example) "EISA Configuration" using 47mb DOS FAT.

The MBR is written when your OS formats the drive. The PPT is how your OS keeps the data on ALL drive partitions on your system. Each HDD gets its own PPT that tells the OS how that drive is partitioned. The simplest is, of course, a single partition.

But if a HDD has been partitioned to (example) 2 partitions, there would be 2 entries in the PPT with the data on each partition. This includes if the other partitions are for a different OS. Example; 1st partition WinXP and 2nd partition Vista (aka you have setup a Dual Boot system).

Physically HDDs are divided into Platters, Tracks, and Sectors.
  • Platters: Disks that have 2 sides (upper/lower) with a magnetic coating that data is written/read

  • Tacks: Concentric circles where data is written/read

  • Sectors: Think pie wedges, this divides each track

Since each Platter has 2 surfaces (upper/lower) Tracks are referred to as Cylinders (like a tin can). All upper tracks align with all lower tracks; and on HDD with more than one Platter, all upper tracks align with all lower tracks, on all Platters.

The HDD keeps track (has a physical address or LBA) of each location on the HDD by Platter/Track/Cylinder. It is this address that is passed to your OS. There are reserved HDD addresses, such as LBA0 where the MBR is located. It is the LBA that is passed to your OS to track HDD contents.

SIDE NOTE: This deals with the term "Partition" when use for HDD vs how the term is used in Mainframe Computers.
  • In Mainframes the term "Partitions" refers to how HARDWARE RESOURCES are allocated. In this context, a Partition can refer to a number of processors used, how many memory DIMMs are used, how many of the Storage Devices are used, how many Network Cards are used, etc.

  • The Storage Devices usually mean "packages" or sets of individual HDDs. Multiple HDDs can be "partitioned" to function as a single Logical Hard Drive (LHD), make 5 HDDs = 1 LHD. Note that when Windows refers to C: this is the LHD alias for HDD0, 1st Partition.

I will post about the NTFS (NT File System) use by WinXP & Vista later.

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