Unable to create .vmdk file for USB disk on windows host

[WIlliam_Fraser]

I'm running on Windows XP host (service pack 2) with VirtualBox 1.6.2

According to Windows disk management my hard drives are PhysicalDrives 0 and 1, I've got 2 card readers mounted at 2 and 3, and my USB drive is mounted at PhysicalDrive4

I've tried to create a vmdk file of my USB disk so that I can boot from it. Using the command

VBoxManage internalcommands createrawvmdk -filename D:\disks\USB.vmdk -rawdisk \.\PhysicalDrive0

(or 1) at the command line works with the message:

RAW host disk access VMDK file D:\disks\USB.vmdk created successfully.

And the file USB.vmdk has been created.

If I try the same for a non-existsnt drive for example

VBoxManage internalcommands createrawvmdk -filename D:\disks\USB.vmdk -rawdisk \.\PhysicalDrive2

I get the following error message:

Error while creating the raw disk VMDK: VERR_INVALID_PARAMETER

... So much so good.

However when I try to do the same with my USB disk with the command

VBoxManage internalcommands createrawvmdk -filename D:\disks\USB.vmdk -rawdisk \.\PhysicalDrive4

I get no error messages at all but don't get a sucess message either, just the standard text:

VirtualBox Command Line Management Interface Version 1.6.2
(C) 2005-2008 Sun Microsystems, Inc.
All rights reserved.

The file USB.vmdk is not created

I've looked through the forum and I've noted that someone's noted this problem before with no answers as to how to fix the problem, and I can't find any tickets submitted for this problem. I'm assuming it's a bug and not something I'm doing wrong that I can't find documented anywhere.