I have been backing up for about a year using an external 1TB USB hard drive. I just purchased an Airport Extreme and want to use the same hard drive with the Airport using Airport Disk. What I was hoping was that Time Machine would pick up over WiFi where it left of on USB. When I connect to the Airport Disk, it starts backing up as if there is not already a time machine backup there (though it still shows the same amount of free space). Will I have to erase the drive and start backing up fresh? I'd rather not have to if at all possible.