Harddisk Capacity

[Rubberman]

An extended partition can have logical partitions defined within its boundaries, but it itself is not a partition per se. At least not one that you can format and mount.

The fact that it is big (relatively) and that it is unused are unrelated. If you wanted to access more unused hard drive space, then you could create a logical partition (e.g. /dev/sda5) within the extended partition. You could make it the entire size of the extended partition, or some subset of it. Then you could format it (with ext3, e.g.) and mount it (to /data, e.g.).

To elaborate on this, the default boot loader and partition table must fit on the first sector (512 bytes) of the hard drive typically, although the UEFI firmware interface which succeeds the BIOS is not so limited I believe. In any case, there is only room normally for 4 partitions, but there are times when that is not sufficient, so in came the extended partition idea, which points to an area on the disc where more partition information can be contained. In your case, you have 3 regular partitions, and 1 unused partition, which is configured as an extended partition which can point to additional partitions, although you can change that with the fdisk command to be a regular partition if you wish. Are we confused yet?

[Rubberman]

FWIW, here are a couple of wikipedia links that helps explain it all - one about the Master Boot Record (that first sector on the disc), and one about extended boot records (extended partitions):

Master boot record - Wikipedia, the free encyclopedia
Extended boot record - Wikipedia, the free encyclopedia

[bram88]

is there any information related to used harddisk space in the fdisk command ? or should i combine it with df -h command ?

[Rubberman]

is there any information related to used harddisk space in the fdisk command ? or should i combine it with df -h command ?

The fdisk -l command will show the raw capacities of the disc and each partition, but not maximum or available space in the file systems. For that, you use the df command. Bear in mind that it will show available capacity to the user somewhat less than the maximum since the operating system will reserve some percentage of the total for admin purposes.