How do you do this?
I use VMWare Workstation for testing my USB key.
How do you map the USB key?
Do you map physical USB device to a virtual hard disk?
Which virtual hard disk do you use? Is this the first hard disk?
Which other virtual devices are available at virtual machine?
Do you map USB key as a USB device and boot PloP Boot Manager from floppy image file?
Try this approach.
Try real hardware too.