Have the same issue on same device. It’s because USB3 generates RF noise in Wifi 2.4GHz range.
http://forum.mikrotik.com/t/wifi-connection-deteriorated-with-usb-device-attached/172822/1
I’ve put usb cable and disk itself in aluminum foil to check if it’s related to RF noise and it’s helps. Looking for solution how to shield the disk in more straight way)