hi I'm going to buy the new 15 inch mbp with a 500gb hd and its going to be my first mac. I have a 500gb external hard drive that I'm am going to partition for time machine. my question is now how much space do I reserve for time machine? its because I have allot of data stored on my 2th external hard drive and id like to postpone buying a new external hard drive for as long as possible so being able to still put data on that hard drive would be a big plus. what would be recommended as space reserved for backup? and is there any way for me to exclude curtain maps because I do allot of transferring from laptop to external and to other computers so the files are on my computer very briefly and not worth the space on my backup disk.