I discovered this and thought people might need to know about this.
I am running VM on Mac OS X and I had shared/mapped 2 drives E and F but when I plugged in a usb it didn't show up.
I finally unmapped the drives while the usb drive was connected and it magically connected/appeared as E.
I remapped the shared folders to X and Y so I don't think it will happen again.
I hope this was useful