How to use a real partition with Windows 7 installed, in a virtualbox vm?

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http://askubuntu.com – My Ubuntu 12.04 install is on /dev/sda5 and Windows 7 is on /dev/sda1. When I am running Ubuntu, I would like to use Virtualbox to run Windows 7 in a VM. The Windows 7 partition is 1 TB and is half full, i.e. large, so I don't want to copy it into a virtual hard disk. I have read that it is possible to use a real hard disk for a Virtualbox VM, but the various instructions I have found differ from each other, and I can't seem to get it to work. Does anyone know a way to make it work in Ubuntu 12.04 and Virtualbox 2.1.12_Ubuntu r77245 (the latest Ubuntu installed the repos)? Please post (HowTos)