Open up a Command Prompt and type:
VBoxManage.exe convertfromraw \\.\PhysicalDrive2 C:\output.vdi --format VDI
Replacing "2" with the drive index you want to clone and changing the output location to whereever you want.
That should be it!
Update:
VBoxManage creates a full clone, meaning the image will take up as much space as the physical drive. If you want an image based on just the content, use Disk2vhd from Microsoft
