I had a Seagate external drive connected to my Airport Extreme (via USB hub) for my Time Machine backups. I started frequently getting the message that Time Machine had completed verification of my backups and it needed to create new backups, removing the old ones. I did this a few times and then it started happening constantly so I figured the drive was failing and I bought a new WD drive. It's been working fine for 2 weens now and today I just got the message again. I wonder what's going on?