i have a 250 gig external for TM right now. i currently only have about 90 gb left. im going to be getting a digital camera soon and would like some more space to put them on instead of my computer HDD because that is getting low as well. can i erase my External so there is nothing on there, then have TM make a backup from now? i know i won't have any of the old stuff, but i haven't gotten rid of anything i don't need right now.