done, thanks both of you.
Tell you the truth, some issue still persists and it is not a Shared Folder conflict matter, since I've kept it turned off while testing the USB functionality.
After opened the USB volume and worked with its contents, even only subfolders opening/closing, then the system shut-down often fails, I've to force the guest machine shut down from the VBox menu. Restarting it, the USB volume cannot be used, I've to do the CHKDSK /F to have it working again. It seems like WPS goes in trouble after having browsed the USB volume, even if everything appears ok during its use.