Hi I have got my mac and a couple of external drives now for back up.. After setting up time machine to use one of the externals as the back up.. The other drive is used for working off.. Ok time machine seems to back up both my external drive that I work off and the Mac HD drive. I wanted to test if Time machine works with a simulated failure of the external drive so I dis mounted the external drive (turned it off) .. then started time machine to see if i could find one of the files that i had on there. . Well I could not find the external drive in Time machine.. There was no showing of the drive in the left window of spotlight.. which means to me that time machine will not work if the drive fails. have i got it set wrong?