Stuck copying files

I was copying files from my hard drive to an external drive. (Seagate Backup Plus for Mac). It juwindow.st went on and on without anything from happening. The new disk showed the files I had selected, but at zero bytes and grayed out.

I tried deleting these files, but it said I couldn't as they were being used

At various times, I relaunched Finder, and rebooted several times.

I also tried to copy a new file.

I tried Diskutility with no luck: I couldn't Unmount it, or erase it, or get FirstAid to fix it.

/stop the copying in process?