I have a 500GB HD on my iMac. Is there any advantage to getting say a 750GB drive or should I just stick with 500GB. The reason I ask is since I would be using the external drive as the back up, I guess I shouldn't be putting anything on it exclusively because that would defeat the purpose of TM. Am I thinking correctly here?? I am new to apple and I have never had an external drive to back anything up before.