By your descriptions is not even clear what are you asking. How to make backup or how to solve the slowness?
Besides that, the description of your problem is missing so many details that it's hard for anyone to help you.
What loads very slow? the movie or the hard drive itself? You say you can't open it, so can you open it and it's slow or you cannot open it at all?
How is it plugged (USB or by network) and to what (PC, tablet) and how are you accessing it?
Is it a "2.5 or "3.5 drive (works with or without electricity)?
What software are you using to play the movie?
Did you try to simply copy the file from the external hard disk to your PC, is it slow then? What is the speed (Kb/sec)?
Let's start with that at least..