I have a iMac 27”, 5K with a 2TB Fusion Drive. I am running my Time Machine backup on an OWC RAID Enclosure with two 4TB hard drives. I am not using the hardware RAID that is part of the OWC enclosure, but the software RAID through the iMac. Every so often I get the error message that the drive has been improperly disconnected. The drive is connected directly to one of the USB 3.0 ports in the back.