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The best solution (in my opinion) is to not use a too large storage system, because if it fails you’ll lost all data. The safest solution is to use many external drives (500 GB max is better in my opinion).
So I suggest to buy 2-3 more 500GB drives. There is actually no need to run all drives at the same time, so you can use only the drive containing the movie you want watch.
If too many drives are cumbersome, a nice (but more costly) solution is to get 2.5" external drives, that doesn’t require an external PSU to work. The drawbacks of this solution, however, are a higher price and a smaller capacity, so you need to buy a larger number of drives, and again this makes this solution more costly