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**Update**

Okay, I gave up on the NTFS thing as I wasn't sure how to get it working and no one could help me. Butttt, I did manage to find a program that allows me to read/write to a mac partitioned drive. Here it is:

http://www.mediafour.com/products/macdrive/

So if you want to use an external USB drive with ur ATV but be able to read/write with ur PC, use the above program.
 
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