How do I combine music folders on 2 external hard drives onto one?

In a failed attempt to copy/backup my music on a second external harddrive, I have noticed that my primary external harddrive no longer functions alone, but needs the 2nd harddrive in order to recognize/play my music. Upon opening the music folders on each drive, I find that some of the songs the folder of the primary harddrive may or may not appear in folder of the backup harddrive. Now, many of the songs that I attempt to play from the primary drive will not play unless the backup harddrive is running. How do I resolve this issue?

Thanks for all help