Have you tried.....
Checking the event viewer to see if there is any reference to a problem with the copy process. If not try it again and when it fails (or whatever it's doing) promptly open event viewer and see if their is anything there maybe an access denied or something.
Maybe try a third party utility, something like TeraCopy
http://codesector.com/teracopy it's free for personal use and will generally give you an idea of why a file was not copied.
Run Check Disk on the USB drive that hosts the files that you are trying to copy from.....
chkdsk X: /R (where X: is the actual drive letter of the problem drive)