Here's a little info on Time Machine's behavior that I haven't found anywhere else. I hope you find it useful. I use Time Machine to back up not only my laptop's internal hard drive but also two external drives -- all three get backed up to a third 1 TB disk. Now the question is this: what will Time Machine do when the backup drive is connected but the two drives it's supposed to back up are not? Will it treat the files on the disconnected drives as no longer existing and act accordingly (as if a folder had been deleted) or will it guess that the drive is simply disconnected and treat those files as still existing but unchanged? Since I did not find the answer to this question anywhere I decided to try it. And the answer is: Time Machine acts smart in this situation. It assumes that unmounted volumes are unchanged and does *not* treat them as if all files had been deleted. Kudos to Apple for anticipating this but thumbs down for not documenting it (at least nowhere I could find).