I've replaced and upgraded several hard drives on my old pre-OS X macs, but have yet to do it on an OS X box, but the primary drive on my G4 has been making intermittent funny noises while showing me the spinning rainbow wheel at random times for the past few months. First, is there anything else that could be causing it to do that besides a hard drive on its way out? I'm not totally convinced that it is going bad, but it certainly doesn't make me feel warm and fuzzy when it happens. Is it toast? Could it be something else? If it does need to be replaced, how difficult is it to move everything to a new drive under OS X (I remember just dragging and dropping everything on the drive and renaming the new drive to the old drive's name once the old drive was renamed on pre-OS X systems)?