Difficulty enabling raw partitions

[philipbrown]

I'm following the directions about setting up raw partitions, at
http://www.virtualbox.org/manual/ch09.html#id2685036

The vbox manage command completes successfully:
sudo VBoxManage internalcommands createrawvmdk -filename $HOME/rawpartition.vmdk
-rawdisk /dev/disk0 -partitions 3

(odditity: the command creates TWO files:
rawpartition.vmdk
rawpartition-pt.vmdk)

and then I give myself access to the raw partition, via sudo chown.

I then attempt to "Add" the new vmdk file with the virtual media manager.
BOTH files error out, with:

Failed to open the hard disk
[path to myfile-pt.vmdk]
Could not get the storage format of the medium
[path to myfile-pt.vmdk]
(VERR_NOT_SUPPORTED)

Details:
BOX_E_IPTR_ERROR (0x80BB0005)


What now??