I tend to find USB 3 ports far more flaky than USB 2. My USB 3 drives will connect to USB 2 at all times. But if I've connected and mounted an external drive to the USB 3 port and then remove it safely by "ejecting" the drive, I can't connect other drives until I reboot using the USB 3 port. I can, however, connect the drive to the USB 2 port. Device Manager shows no problems and all ports ready to go.