Here's an interesting question that I have been scouring the interwebs looking for an answer for but have had no luck. Here's my setup. Macbook Pro Retina (Late 2013 15") 4TB External USB Drive Airport Extreme AC (6th Gen. Tower) I made the initial backup via USB since it would take a while over the network (1 TB with other external drive plugged into rMBP). I then go and plug the Time Machine into the Airport Extreme's USB slot. I go to make a back and it doesn't do it incrementally, but treats it as a separate drive altogether. Has anyone dealt with this and figured out how to let Time Machine know that it's the same disk? Thanks!