I actually had no idea this was possible until today when I found out through something else I was looking for. Everything seems to be up to date for me and my software, but for some reason, Time Machine won't recognize an external disk connected to Airport Extreme. When I click on Airport Utility after connecting my 500GB external disk to my Airport Extreme, it lists it under Disks, but then there's no way for me to Time Machine to find/recognize that connection. Maybe I'm doing something wrong. Any help would be great. I really need the USB slot in the back of the iMac that the external disk is using.