Hi
I have 3 visible partitions on my hard disk - one for windows vista, one for my data and one for linux. One other is swap partition. fdisk prints the following partition table:
---------------------------------------------------------------------
Disk /dev/sda: 160.0 GB, 160041885696 bytes
255 heads, 63 sectors/track, 19457 cylinders
Units = cylinders of 16065 * 512 = 8225280 bytes
Disk identifier: 0xb0000000

Device Boot Start End Blocks Id System
/dev/sda1 1 9 72261 de Dell Utility
/dev/sda2 10 1967 15727635 5 Extended
/dev/sda3 * 1968 6537 36700160 7 HPFS/NTFS
/dev/sda4 6537 19458 103787520 7 HPFS/NTFS
/dev/sda5 10 33 192748+ 82 Linux swap / Solaris
/dev/sda6 34 1967 15533056 7 HPFS/NTFS
----------------------------------------------------------------------

As u can see, the only extended partition is /dev/sda2.
but cmd - sudo fdisk /dev/sda2 returns the message: unable to read /dev/sda2.
I wanted to create a new minix version 1 partition out of it for testing some old kernel.
How to get over this error?

You are using fdisk incorrectly - it is only used with devices (/dev/sda) not partitions (/dev/sda2).

Invisible_Coward:

First of all you don't fdisk an extended partition. You do it against a device.
Try

:$ sudo /sbin/fdisk -u /dev/sda

press m for help, there you can do anything against your hard drive BUT are you sure how much space is available and which sector?

But why use fdisk? Use your gParted or other GUI tool to be sure of what you are doing.

Also try sudo /sbin/cfdisk /dev/hda (or /dev/sda) be sure issue this outside X. This one has better dialogue choices. Read help first:
$ man cfdisk

Goodluck.

fdisk works on drives, not partitions. If you wanted to create a new partition on the drive, you would do fdisk /dev/sda. Then you would be able to create a new partition out of whatever is left. But, from what I'm seeing, the sda2 extended partition (if the intention is to create a new partition inside of it) is full.