I have a Win 8.1 laptop. I also have a HDD in an external docking unit connected by USB3 and am trying to copy from it to the internal HDD on the laptop. The transfer speed is very variable and most of the time abysmal. For short bursts it whizzes along at 30-50 MB/s then drops for long periods to crawl along at around 100 kB/s or less. I am not doing anything else significant, so why the wide speed variations I wonder. Any ideas?