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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 with any desired file system.

Once a disk partition has been logically formatted, it is referred to as a volume. As part of the formatting operation, you are asked to give the partition a name, called the "volume label." This name helps you easily identify the volume.

Why Use Multiple Partitions?
Many hard disks are formatted as one large partition and this does not always provide the best possible use of your disk space or resources. The alternative is to separate your hard disk into partitions.

With multiple partitions, you can
Install more than one OS on your hard disk
Use your available disk space more efficiently
Make your files as secure as possible
Physically separate data so that it is easy to find files and back up

If you have multiple primary partitions on your hard disk, only one primary partition may be visible and active at a time.

The active partition is the partition from which an OS is booted at computer start-up.

Primary partitions other than the active partition are hidden, preventing their data from being accessed; thus, the data in a primary partition can be accessed (for all practical purposes) only by the OS installed on that partition.

If you plan to install more than one operating system on your hard disk, you probably need to create multiple primary partitions; most operating systems can be booted only from a primary partition.

Extended Partitions

The extended partition was invented as a way of getting around the arbitrary four-partition limit. An extended partition is essentially a container in which you can further physically divide your disk space by creating an unlimited number of logical partitions.

An extended partition does not directly hold data. You must create logical partitions within the extended partition in order to store data. Once created, logical partitions must be logically formatted, but each can use a different file system.

Logical Partitions

Logical partitions may exist only within an extended partition and are meant to contain only data files and operating systems that can be booted from a logical partition (for example, Linux, Windows NT, etc.)

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 to boot from.

The primary partition that the computer boots from is called the active partition. If there is not an 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.

Partition Genius's Set Active lets you easily choose which primary partition you want to be the active partition.

Making Good Use of Logical Partitions
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

Making Efficient Use of Disk Space
If you have a large hard disk and want to use the FAT file system on all or most of the disk, you can prevent wasted space by using several small FAT partitions. All data on a FAT partition are stored in allocation units called clusters. Each cluster is made up of a fixed number of disk sectors.

The FAT file system supports disk or partition sizes up to 4 GB(Windows XP), but only allows a maximum of 65,525 clusters. Therefore, whatever the size of the hard disk or partition, the number of sectors in one cluster must be large enough so that all available space can be included within 65,525 clusters. The larger the available space, the larger the cluster size must be.

However, using a large cluster size wastes disk space. Even if a data file (or the last portion of a data file) is much smaller than the cluster size, the computer must still use a complete cluster to store the data. The rest of the cluster space goes unused. The following table shows the minimum cluster size and typical wasted space for various partition sizes.