I have an ASUS laptop with a factory installed version of Windows 7 (64-bit) on the internal hard drive, and CentOS 6.4 running off an external USB drive.
My goal was to use Windows 7 to host as many browsers as possible for testing. For that reason, I needed to be able to have both Windows and CentOS running at the same time.
I used the following commands to map the Windows drive for use with VirtualBox:
As the regular user:
Then I created the virtual machine and assigned it to use the VMDK and booted it up.