If you copy your music to an external HD, be sure and format the drive in a Windows readable format. Don’t format it for a Mac. Format it in the Mac Disk Utility and chose the format that specifies it will be readable by both the Mac and a Windows computer. . . otherwise, your computer will not be able to not read your music on it.
That really helps me, as I tried this previously without success. That may be where I went wrong.