Does Apple TV only take videos from iTunes, or does it act like an external Hard Drive? Most of my videos are not in iTunes they are on my MacBook Pro's Hard Drive or my External Hard Drive. Can I drag and drop videos into Apple TV?
Apple TV not only doesn't work like an external HD, it works more like an iPod or iPhone -- whatever is iTunes on your computer
can be synced with (copied to) the Apple TV.
In addition, you have the option of streaming content from iTunes to the Apple TV without having to sync it.
Your concern about adding movies to your iTunes is a non-starter. By holding down the "Option" key while dragging a movie, and dropping it on your iTunes library, iTunes will leave the original file where it was. iTunes will now have made "an alias/a reference" to this original file, sitting on an external drive or anywhere else. So, as long as iTunes can see this file, your Apple TV can, too.
iTunes acts as a "server" of sorts, and as long as it is running, all of its content (whether aliased or copied locally) is available for syncing OR streaming to an Apple TV.
So, you control which movies (if any) get synced, and those that don't get synced can be streamed. This control is set up through iTunes on your computer.
In addition, Apple TV can directly search for and download podcasts, etc. without your computer needing to be on. You can rent movies without iTunes, too.
Hope this helps answer your real question.