Having trouble to partition my hard drive during Red Hat Installation

[Diego del Río <diegodelrio hotmail com>]

I'm having problems to make the hard disk partitions when installing Red Hat
7.
I have a Western Digital 20 GB hard disk (model WD200BB), it is partitioned
following this schema:
    1GB Primary Dos Partition
    2GB Extended Dos Partition
    13 GB NTFS Partition
    3.1 GB Unallocated

I'd like to install linux using the unpartitioned space. During the
installation, i choose to define the partitions manually using fdisk or
diskdruid, but with both of them i failed when i tried to add the second
partition. I got the message "The partition you asked to add could not be
allocated. No free primary"

Has anybody experienced something similar.

Thanks in advanced,

Diego