My partition table is facing limitations; having problems creating more partitions

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http://askubuntu.com – I first noticed an issue when trying to install Linux Mint 14 as a third OS alongside Ubuntu 12.10 and Windows 7 - I was unable to create another partition to install Mint to. Poking around, I realised that I had reached the limit of primary partitions: (from left to right of the table) 1) a ~100 MB primary partition that I meant to use for storing Grub files but never got down to, 2) a 25 GB extended partition, with the entire partition containing an ext4 Ubuntu partition, 3) a ~513 GB partition containing Windows 7, 4) some ~50 GB of unallocated space, and finally 5) a 4 GB swap partition. (HowTos)