Adding some infos I had to research for a similar task and adding to the answer of gcb, where using usb is not an option. So make sure to read his post first. (i cannot comment his post)
Since createrawvmdk is deprecated you can use createmedium instead, that has this syntax and creates a vmdk-file pointing to the physical Drive:
VBoxManage createmedium disk --filename <path_and_filename>.vmdk --format=VMDK --variant RawDisk --property RawDrive=\\.\PhysicalDrive123
On Windows you'll find your PhysicalDrive-Number by opening Disk Management (i.e. PhysicalDrive2).
On linux it'll be "sudo fdisk -l" and the property "RawDrive=/dev/sda".
For MacOS have a look on http://www.virtualbox.org/manual/ch09.html#rawdisk
Under Windows, it's possible that you'll get some Errors like VBOX_E_FILE_ERROR / VERR_ACCESS_DENIED / etc., when the VBox-service hasn't started with enough privileges. It can be enough (it was for me) to stop the service "VBoxSDS" through the task-manager and let the VirtualBox-Manager start it automatically by calling the command mentioned above. For some others i read that they had to specifically start the service with the right account.
I hope that answer shortens the search for other users.