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What is the difference between "extended" partitions and "logical" partitions on my hard disk? What's the need for each? I am using Linux

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Historically, hard drives have only been able to contain at most four partitions because of the originally defined format of the partition table. This is not specific to operating systems. You simply can't create more than four primary partitions.*

In order to circumvent this limit and still remain compatible with older systems, you can create an extended partition however. An extended partition can contain multiple logical partitions within it. This allows you to create more than four partitions in total, without having to change the format of the partition table.

If you're interested in the details, you can look at the Wikipedia entries on disk partitioning or the master boot record.


* At least on computers with BIOS firmware, I don't know if the UEFI standard (a BIOS-replacement) lifts this limitation.

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More precisely: an extended partition contains itself again a partition table, which can hold up to four partitions, each of which can in turn be an extended partition which holds a partition table ... leading to a 4-ary tree of partition tables. Or actually, a linked list, since DOS itself only supports one primary and one extended partition per partition table. In other words: an extended partition is a primary partition which itself contains another partition table, a logical partition is a primary partition which does not sit in the root partition table. –  Jörg W Mittag Feb 20 '11 at 14:48
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EFI can use either the legacy DOS MBR scheme and also a new scheme called GPT which does not have this limitation. –  mattdm Feb 20 '11 at 21:01
    
If you plan to run other operating systems on the same hd, during setup it's best to create just one primary partition and use extended for the rest. Some other OS's won't install if there's isn't one free primary partition. –  ultrasawblade Feb 21 '11 at 0:30
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