Hi. I have 2 external HDs, one static, one portable. The portable one I use for a portable photo/music Library to extend the limited space of my MacBook Air. The static one is plugged to my iMac and is a Time Machine backup for the iMac and the portable HD (I removed the portable HD from the "exclusion list" in Time Machine so it's included in the backup). Since the portable HD is not always plugged to the iMac, Time Machine sometimes backup only the iMac. When I plug in the portable HD once in a while, it includes it in the backup. However, and this is where my question is, it starts an entirely new backup (of like 700 GB) instead of backing up only the changes made since the last backup (that included the portable HD). Is there a way to avoid this ? If not, I guess Time Machine isn't yet a great tool for backing up portable HDs... ? Thanks you.