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Partitioning Basics

What is a hard disk?

A hard disk/drive is one the part of your computer responsible for long-term storage of information. Unlike volatile memory (often refers to RAM), which lost its storage information once its power supply is shut off, a hard disk stores information permanently, allowing you to save program, files or any other data. Hard disks also have much more powerful storage capabilities than RAM.

What is disk formatting?

Computer must be able to access needed information on command; however, even the smallest hard disk can store millions of bits. How does the computer know where to look for the information it needs? To solve this problem, hard disks are organized into discrete, identifiable divisions, thus allowing the computer to find any particular sequence of bits easily.

The most basic form of disk organization called formatting. Formatting prepares the hard disk so that files can be written to the platter and then quickly retrieved when needed. Hard disk can be formatted in two ways: physically and logically.

Understanding the file systems

All file systems consist of structures necessary for storing and managing data. These structures typically include an operating system boot record, directories, and files. A file system also performs three main functions: 1) tracking allocated and unused space, 2) maintaining directories and filenames.3) tracking where each file is physically stored on the disk.

Different file systems are used by different operating systems. Some operating systems can recognize only one file system, while others can recognize several. Some of the most common file systems are: FAT (File allocation table), FAT32 (File allocation table 32), NTFS (New technology file system), Linux ext2 and Linux swap.

Understanding partitions

After a disk has been physically formatted, it can be divided into separate physical sections or partitions. Each partition functions as an individual unit, and can be logically formatted by any desired file system. Once a disk partition has been logically formatted, it is referred to as a volume.

During the process of formatting operation, you are asked to name the partition, called the "volume label". This name helps you easily identify the volume.

Understanding how a computer boots

The way a computer boots from a hard disk depends on the way that hard disk is partitioned and the way that operating system is being booted. When you turn on your computer, the central processing unit (CPU) takes control. It immediately executes the instructions built into the computer's ROM BIOS, a program which contains the startup procedures. The last part of the BIOS instructions contains the boot routine. This routine is programmed to read the master boot record (MBR) from the first sector of the first physical hard disk.

The MBR contains a master boot program and a partition table which describes all of the hard disk's partitions. The BIOS boot routine executes the master boot program, which then continues the boot process. The master boot program looks over the partition table to see which primary partition is active. If there is only one primary partition, then the partition's OS is loaded and booted into operation.

Managing your partitions

The following concepts and activities can help you manage your disk partitions.

  • Setting an active primary (boot) partition. When you create multiple primary partitions to hold different operating systems, you must tell the computer which primary partition is to boot from. The primary partition from which the computer boots is called the active partition. If there is no active primary partition on the first physical hard disk, your computer will not be able to boot from your hard disk.
  • Before you make a primary partition active, make sure that it is a bootable partition. Bootable partitions are logically formatted and have the necessary OS files installed. Partitions without an OS cannot be booted.

By creating an extended partition and then dividing it into logical partitions, you can:

  • Access the Same Files from Multiple Operating Systems
  • Make Efficient Use of Disk Space
  • Simplify File Access and Enhance File Security

Understanding drive letters

The OS that you boot assigns drive letters to the primary and logical partitions on each hard disk. These drive letters are used by you, your system, and all your applications to reference files on the partition.

Your OS may change the drive letter assignments if you add or remove a second hard disk. Drive letter assignments may also be altered if you add, remove, or copy a disk partition; reformat a partition with a different file system; or boot a different OS. These kinds of drive letter changes can sometimes invalidate parts of your system configuration. For example, applications that are programmed to look for startup files on a specific drive may no longer launch.

Understanding the BIOS 1024 cylinder limit

The BIOS 1024 cylinder limitation exists because the start and end cylinder values on the partition table (and some BIOS's) have the maximum value of 1024. Because some operating systems such as DOS 6.22 use the CHS (Cylinder, Head, and Sector) values to address sectors on the disk, they cannot access sectors beyond the 1024 cylinder. When you start your computer the BIOS boots the operating system using the CHS values to locate the first sector of the bootable partition. If the partition starts beyond the 1024 cylinder, the BIOS may not be able to boot it because it cannot address a cylinder number higher than 1024.

Changing the BIOS LBA mode setting

Caution: Never change the LBA mode in your system BIOS once data is present on any hard disk. Changing this setting may cause data corruption and loss.

Most modern system BIOS designs support LBA or Logical Block Addressing. The LBA mode setting, no matter enabled on your system or not, determines how your computer translates logical cylinder-head-sector (CHS) addresses. If you change this setting, the resulting shift in CHS values may corrupt all the files and partitions on your hard disk(s).

If you must change the LBA mode setting in your system BIOS, first back up all data on your hard disk(s). Contact the BIOS and/or disk manufacturer's technical support departments to ensure that understand how to proceed safely.

Understanding the 2 GB boot code boundary

An operating system's boot code is stored in both the master boot record (MBR) and the partition boot record, enabling the OS to boot properly. In some operating systems, however, this boot code is written is such a way that it inadvertently imposes a limit on the location of both the partition boot record and the files needed to boot the OS.

DOS (versions 6.x and earlier) and Windows NT (versions 4.0 and earlier) are both affected by this boot code limitation. When booting these two operating systems, the cylinder-head-sector (CHS) address of the beginning boot code sector must be calculated in order to retrieve the sector's information, thus to load and execute the next part of the boot process. The CHS value of for the needed sector is calculated as follows:

Sector Number / Sectors per Track

Because of the way the boot code is written, the product of this calculation must fit in a 16-bit register. The largest value of 16-bit register may contain is 64K. If the value is larger than 64K, the number is truncated, resulting in an incorrect value that skews the remaining calculations. The boot process fails to load and execute the needed sector, thus prevents the OS from booting.

Most current hard disks have 63 sectors per track, creating a 64K boot code boundary at 2 GB. If your disk is older and/or uses drive overlay software, this boundary may be lower.

If a partition begins or extends beyond this boundary, the CHS value of the partition's boot code sector cannot be correctly calculated; therefore, the partition and its OS cannot boot.

The same limitation applies to the DOS IO.SYS file and the Windows NT file, NTLOADER.EXE. Either of these files being installed or moved beyond the 64K boot code boundary may cause the corresponding OS fails to boot. To boot properly, DOS requires that the first three sectors of IO.SYS are below the 2 GB boot code boundary.

Both the IO.SYS and NTLOADER.EXE files are usually located near the beginning of the partition in which they are installed. When you use EaseUS Partition Master to resize partitions, this area of the partition may be vacated to make room for a larger FAT or other file system structures. As a result, IO.SYS or NTLOADER.EXE may be moved beyond the 2 GB boundary, thus preventing the OS from booting.