How Do I Get the iTunes On My iMac To Play the Media On My External Drive?

I have all of my media on my external Seagate drive. I can’t get iTunes on my iMac to “see” the media. I’ve tried to redirect iTunes to the drive but I can’t make it play the music, movies or tv shows. It used to work but I screwed it up some way. I didn’t sign up for Music or anything.
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Tom Brandon