Confirm.
+Attach you hard Drive to MAC (do not assign to OS, leave it unmounted)
+Create a new VM , choosing the final OS your external USB hard drive will boot.
+Choose to 'customize' you VM after creation.
+Remove the Hard Drive created by default
+Add a new Hard Drive of type 'Boot Camp'
+Location should show your attached bootable USB drive , choose it
+Customize the other settings as needed (Net,Memory..)
+Boot it.
Note: it may be needed to change the 'Location' of your drive from
IDE,SATA,SCSI...
Please post your findings.