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 root:

chmod a+rw /dev/sda

As the regular user:

VBoxManage internalcommands createrawvmdk -filename /opt/vbox.disks/windows7.vmdk -rawdisk /dev/sda
VBoxManage storagectl 'Windows 7' --name 'SATA' --hostiocache on

Then I created the virtual machine and assigned it to use the VMDK and booted it up.