find the usb disk number with diskmgmt.msc, eg: disk 1 or disk2,
cd %programfiles%\Oracle\VirtualBox
VBoxManage internalcommands createrawvmdk -filename C:\usb.vmdk -rawdisk \\.\PhysicalDrive#
eg:
VBoxManage internalcommands createrawvmdk -filename C:\usb.vmdk -rawdisk \\.\PhysicalDrive1
