If you're using the USB connection on the CK3 Pro, the loopback test won't actually prove anything (sorry guys). The loopback test is for testing COM ports on computers, whereas this is just a USB to USB connection.
Double check in device manager to ensure you have the correct COM port (I'm sure you've already done this).
It's already been asked by chorizo1, but ensure the drive is in the correct configuration:
Push the eject button on the CK3 to eject the tray.
Push it again but turn off the CK3 before it closes.
Turn the CK3 on again (tray should still be half-out).
You need the drive like this for dumping the key. If it fails, you need to repeat the steps to put it back into the right mode.