OK, thanks for the additional information.
Your process may have worked in the past, but that is actually the way to copy items in Windows. On a Mac, the normal way to copy files from one drive to another is to simply click and hold on the items, and drag-and-drop them to the destination, between two Finder windows for example. And, if you hold the Option button, it will make a copy on the same drive, while leaving the original intact.
Maybe the issue this time is the way the USB hard drive is formatted, possibly for Windows?
Strange issue.
C