I'm going to upgrade my Core Duo Macbook Pro in the next few days with a new 320gb HDD. My external disk for which also includes my Time Machine Backup is only 250GB. Obviously one is larger than the other, will this cause a problem? If so is it possible for me to put a limit on how much space Time Machine can take up? If for example I tell it to exclude /System and /Library from the backup can I still in future when install a fresh copy of OS X, restore from a Time Machine backup?