I have a fairly new iMac that I upgraded to Snow Leopard a few months ago. It seemed to me that prior to that my external HD worked fine with Time Machine. Since then it is recognized for a short period of time when I connect it, then it disappears. I replaced it with a WD My Book. It worked perfectly for a month and now it has shut down. Has anybody had run into this before? I can't believe that every hard drive fails within a month in the same mysterious way!