This is how I did it in VitualBox under Windows:
Go to the VirtualBox directory under c:\Program Files\Sun and run:
VBoxManage.exe internalcommands createrawvmdk -filename \usb.vmdk -rawdisk \\.\PhysicalDrive1 -register
This creates a virtual rawdisk pointing to PhysicalDrive1 on your Windows machine (hopefully your USB drive), check the position of your drive (the number) in Windows Disk Management. The option "-register" adds the newly created drive (usb.vmdk) as a drive option available to any machine in VB.
Have Fun!
