I hope it's okay to start a new thread based on my previous thread of the same title, because I've discovered that the problem is with FreeFileSync. There are many forum posts on their forum, about people having slow transfers with FreeFileSync, some having the 'zero bytes' complete stop that I've been experiencing. When I copy the same files in FreeCommander or Windows Explorer, I get 60MB/s consistently. No going up and down, no variation, no going down to zero, no problems at all. So I'm copying all the folders over manually with FreeCommander, then I'll only use FreeFileSync to check that both drives have the same files, and allow it to do any further copying (which is normally only 50GB or thereabouts.)