Hello all. I'm new to the mac platform, and I'm trying to set up time machine. So, i open up the time machine program, and I'm presented with a switch to turn time machine on and off. and when I turn it on, it brings up a "Drive select" screen. Now, the problem is, I have a 500 GB HDD attatched via USB, but it doesn't appear. So....I have a external harddrive attached, Time machine won't allow me to back up to it. What do I do?