I have just picked up a refurb Mac Mini and am using it with a Logitech K400 USB keyboard. It all works fine, until I connect an external HDD. At that point the keyboard stops working, but it is fine again when I remove the HDD! I have tried varying the USB ports I am using for each, but it doesn't seem to be able to support both at once. I don't really want to send it back to Apple if it is a 'feature' rather than a fault, but I have never seen anything like this before.