Understanding Hard Disk PartitionsBy Lawrence Abrams on November 18, 2005 | Last Updated: February 27, 2012 | Read 151,634 times.
Table of Contents
When a hard drive is installed in a computer, it must be partitioned before you can format and use it. Partitioning a drive is when you divide the total storage of a drive into different pieces. These pieces are called partitions. Once a partition is created, it can then be formatted so that it can be used on a computer. When partitions are made, you specify the total amount of storage that you would like to allocate to that partition from the total size of the drive. For example, if you have an 80 GB drive, then it would be possible to make one partition consisting of the entire 80 GB of available storage. Alternatively,you could make two partitions consisting of a 20 GB partition that will be used for the operating system and programs and a 60 GB partition set aside for data, music, and images.
In the current IBM PC architecture, there is a partition table in the drive's Master Boot Record (section of the hard drive that contains the commands necessary to start the operating system), or MBR, that lists information about the partitions on the hard drive. This partition table is then further split into 4 partition table entries, with each entries corresponding to a partition. Due to this it is only possible to have four partitions. These 4 partitions are typically known as primary partitions. To overcome this restriction, system developers decided to add a new type of partition called the extended partition. By replacing one of the four primary partitions with an extended partition, you can then make an additional 24 logical partitions within the extended one. The table below illustrates this.
As you can see, this partition table is broken up into 4 primary partitions. The fourth partition, though, has been flagged as an extended partition. This allows us to make more logical partitions under that extended partition and therefore bypassing the 4 partition limit.
Each hard drive also has one of its possible 4 partitions flagged as an active partition. The active partition is a special flag assigned to only one partition on a hard drive that the Master Boot Record (MBR) uses to boot your computer into an operating system. As only one partition may be set as the active partition, you may be wondering how people can have multiple operating systems installed on different partitions, and yet still be able to use them all. This is accomplished by installing a boot loader in the active partition. When the computer starts, it will read the MBR and determine the partition that is flagged as active. This partition is the one that contains the boot loader. When the operating system boots off of this partition the boot loader will start and allow you to choose which operating systems you would like to boot from.
Now that you know what a partition is, you may be wondering why you would even need to make multiple partitions instead of just making one. Though there are quite a few reasons, we will touch on some of the more important ones below:
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