Apple TV, iTunes and an External Hard Drive

yes it's possible. You won't be able to have iTunes automatically manage the file locations though, you'll have to do it yourself.

You'll also need to have the macbook turned on (and iTunes running) when you want to watch video on the macbook.

 

You'd have to uncheck the "keep library organised" in iTunes preferences... then just drag files from the external drive to iTunes.

Should work with two macbooks, yes. Both would need to add the files to their own respective iTunes libraries though.

 

You will use one MacBook / iTunes for storing and streaming movies, music, etc. then stream to Apple TVs, iPhons, iPods and other MacBooks all you want. The process is called Home Sharing and it can be used to stream your library of content to various devices on the same network. You can also have multiple libraries shared on the network (more than one iTunes library) and all your devices can stream from them.

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I have this setup, movies on external drive music on MacBook pro. It's a bit tedious to manage if you are adding content not from Apple. For my movies I manually copy them to external drive and then add them to iTunes, making sure to uncheck the box for keeping library organized. Music is less work if it comes from iTunes or even Amazon since these will download to the computer and get added straight to iTunes. So unless you're pulling music from a disc or something you should be ok, just watch where it goes and move it if necessary.