Can you not just move your entire itunes movie collection to your external HDD, then delete them from itunes. After this just readd them to itunes from their location on the EXT. HDD (I know that you can do this with windows itunes) Then all you would need to do is when ever you sync to your iPhone, make sure your EXT. HDD is connected up and voila, you're good to go.
With regards to al217739's comment about EXT. HDDs being less reliable, I don't see how you can say this, maybe you've just been unlucky, but, the sheer fact that the mechanical element of the EXT. HDD is used far less frequently means that they are far less likely to die from mechanical failure. That's why most folks use them for backup purposes, as I say, maybe you've just been unlucky.
I myself, have yet to have a single HDD die on me, going back at least 10-12 years, guess I'm just a lucky boy (touch wood)

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