Virtualbox boot (dual-boot) partition

In Ubuntu:
VBoxManage internalcommands createrawvmdk -filename usbdisk.vmdk -rawdisk /dev/sda
sudo chmod 777 /dev/sda
sudo chmod 777 usbdisk.vmdk

In Windows:
VBoxManage internalcommands createrawvmdk -filename "C:\ubuntu.vmdk" -rawdisk "\\.\PhysicalDrive0"