For MacOSX
diskutil list
# find your usb driver, then unmount it, maybe
# we assume it is /dev/disk2 here
diskutil unmountDisk /dev/disk2
# create a raw vmdk
sudo vboxmanage internalcommands createrawvmdk \
-filename ~/usb.vmdk \
-rawdisk /dev/disk2
# unmount again
diskutil unmountDisk /dev/disk2
# open VirtualBox in root
sudo VirtualBox
Select Exist hard disk, attach the ~/usb.vmdk file.
Then start.