Treating a virtual drive as a USB

[Melab]

Note, this is NOT about passing through a USB drive to a guest operating system. What I need to do is create or treat a VDI/VMDK/VHD such that the guest operating system sees it as a USB. This is critically important for what I am doing because it can probably only work if I'm using a USB drive. How do I do this? I know how to do it for a floppy disk, but not USBs.