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When I installed Ubuntu 12.10, I installed it onto a 500 GB hard drive using the default partitioning.

Looking at the hard drive in Disks enter image description here

I see the main partition (/dev/sda1 = 496 GB) and the swap partition (/dev/sda5 = 4.3 GB). I also see another partition (/dev/sda2 = 4.3 GB), which is not mounted. Its type is Extended and its contents are Extended Partition.

Here is the output from ls /dev/sda*:

/dev/sda  /dev/sda1  /dev/sda2  /dev/sda5

Why aren't there partitions 3 & 4? And what is /dev/sda2 for?

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Totally unrelated... but I hope you're not using that drive anymore. It's failing. – gertvdijk Feb 4 '13 at 0:14
up vote 2 down vote accepted

Partitions in the traditional partitioning scheme (MSDOS table, not GPT) is working with primary and extended partitions. One can have four primary partitions at maximum, or three primary and an extended in which the extended partition can hold a few more (inside it).

The kernel represents all primary partitions as 1-4 and all extended from 5 and up. The sda2 you're seeing is the extended "umbrella" partition, which holds just one more partition: sda5. This numbering scheme is just the way it's being represented.

In your case with just two actual partitions it wouldn't have been necessary to create an extended partition - as you're not hitting the amount of primary partitions limit.

Further readings

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