I have nearly the same problems with my 2,5″ hard drive and the mini usb 3 port.
When i want to copy a file to the pc or another drive than the drive disconnects and connects instantly again.
The same thing with video files. I start a video. After a while approximately 5-8 min it stops because the drive disconnects.
Have someone else the same problem with the mini usb 3 Port?
The usp type c port works well with an adapter =)