another problem might be power. I do not know what you have connected to the USB drives, but if you have nothing on the one and it works on that one, but not on the other and you have something like, say a web cam on it, then it may be that the other USB devise is taking too much power away from the backup drive and the external drive is not working for that reason. Do not forget, USB drives are not solo. They use a hub, so if you have 2 USB plugs, they may both use the same power source for the plugs. I have seen times were there is 4 usb plugs on 1 hub which also mean 4 plugs from 1 power source.