As you know, retrieving lost files is more or an art than a science. But, you didn't mention whether or not you tested your external hard drive for integrity. Many times, a failing hard drive, even if still spinning can be difficult to recover using Windows tools such as Recuva or GetBack; both tools we highly recommend here on WF. Linux tools such as
TESTDISK or
PHOTOREC are also good, as they can work on partially failing drives. Also,
SPINRITE is another good tool if your drive has only partially failed, as it attempts to repair the damage on the drive and then tries to recover data.
Check my
TROUBLESHOOTING GUIDE here for hard drive and RAM Stick testing procedures:
Windows 10 - Unclickable Task...