can you run movies off external drive

Drag the entire iTunes library to the external drive, launch iTunes with the Option key held down, choose that library, and back it up to a second external drive.

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Just so it is clear, the "library" to which Niel refers when he makes this post is the entire iTunes folder, not just the .itl library file. Additionally, some people say you now have to select the .itl file when selecting a new library, not just the library folder.

Moving a library: Quick answer if you use iTunes' default preferences settings: Copy the entire iTunes folder (and in doing so all its sub-folders and files) intact to the other drive, hold down the option/alt key while starting iTunes, and when requested to select a library select the iTunes Library.itl file in the copied folder.

The above process will move the entire iTunes library to the external drive. It is possible to move movie files only but it is an ongoing process as there is no preference setting for doing this automatically for videos, nor to have only videos saved to an external drive.