Given: On a Win-8.1-64bit host, I attached old harddisks by a usb-to-ata adapter (manufacturer LogiLink).
Problem: When I assign the usb adapter to a win-8.1-64bit guest, the attached drives disappear on the host, but do not appear on the guest machine. In the device manager of the guest, the usb device is not even listed. For comparison: The same usb device can be successfully assigned to a win-8.1-32bit guest or any other 32bit guest.
What might be missing in the 64bit guest concerned?