I have Mac Pro with with 1.5TB of hd space installed (soon to grow to 2TB), roughly 50% full. So what happens if I designate a 500 GB external drive as my TM? Obviously stuff will get dropped (backup size > TM), so it seems to me that the whole TM idea blows if you have more than 1TB GB hd space installed in you machine (I sure as hell don't want to spend hundreds of $$ for a 2TB external drive just to use it as a TM).