Restarting Windows Explorer may help you stop the generic volume device. You will also get the "Windows can't stop your 'Generic volume' device..." error if your antivirus is scanning it. Let your antivirus finishes the scan. After that, you will be able to remove the device safely. But if you are in a hurry, you can stop your Generic volume device by terminating the antivirus scan.
If you have opened multiple programs, one of those programs might be using the Generic volume device. In this case, closing the running programs will stop the Generic volume device.