I have several External Drive docking units. Been fine for long time. Now, they seem to eject on their own and I get pop-up on Imac that drive was ejected improperly. I can unplug or turn off/on and it will work for a while. I have switched hard drives from one dock to the other with the issue staying with the dock. Docks are USpeed 6618 & ANKER 6618 USB-2. Both run thru a USB powered hub. What could cause the random ejection? Suggestions?