Ok, to make the 3rd-level backups I keep offsite, I end up copying a single large (1.2TB) file from one USB drive to another...typically takes about 3 hours and change. About half the time, at some point during the process, it will pause as shown below. I can resume it, sometimes it will finish, sometime pause again, never fails to (eventually) copy the file. It does not seem to be correlated to whether I'm using the computer during the copy (I usually don't), and I've tried all the obvious power settings, including preventing screen shut-off. (And no, the computer is not going to sleep.)

Win10, all updates, i7 desktop. Any clue what's going on?
Copy pause.jpg