From your post I see you have 2 copies. The one on the internal and another on the external. So if either go missing you don't lose a thing.
Before you delete the internal HDD copy, you don't want to learn about external drives and reliability the hard way.
Bob
Once I have moved things to my external hard drive (say, photoshop or itunes), do I delete these from my computer?
In other words, it is not clear to me how an external hard drive frees up space.

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