How do I/can I change my location of my Hard Drive with my Time Machine backups form a USB attached to my iMac to my Airport Extreme to back up my iMac and rMBP without losing my iMac backups. iMac has been backed up for over a year. Currently it has the back ups on there and can access them when I go through the disk. When I go to set up Time Machine it shows that there never was a back up and wants to recopy everything over. Thanks!