appletrip

Hi
I have macbook with 150gb hard drive, which I have until now used to download itunes content - mainly tv shows and music - my hard drive is now full. I have resolved to buy an external hard drive with an additional 165gb. I have just completed moving my itunes folders onto this external hard drive. My question is then - when i want to download more itunes content where will it go - the new external hard drive or into my itunes folder on the macbook? Further when i sync with my apple tvs - i have four - will they know where to find the itunes material which is now on the hard drive. Again further - what happens when the apple tvs are full - as theoreticallly if i sync everything they will be - can you link up another apple tv to an existing one to expand its capacity? 160gb really is not much these days, considering that apple forces you to keep all the content that you buy on a drive of some kind - due to the fact that once you delete it you cannont ask itunnes to let you download it again - my questions are all about space management and keeping apple tv and all ones computers synced - its a great device but the constraints of itunes rules and small hard drives are concerning to me.

thanks your help.

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appletrip