To create an image that represents an entire physical hard disk (which will not contain any actual data, as this will all be stored on the physical disk), on a Linux host, use the command
VBoxManage internalcommands createrawvmdk -filename /path/to/file.vmdk
-rawdisk /dev/sda
This creates the image /path/to/file.vmdk (must be absolute), and all data will be read and written from /dev/sda.
On a Windows host, instead of the above device specification, use e.g. \\.\PhysicalDrive0. On a Mac OS X host, instead of the above device specification use e.g. /dev/disk1. Note that on OS X you can only get access to an entire disk if no volume is mounted from it.
