Original post by: fodriscoll ,
Yes, you can. The A1409 contains a standard WDC Green 2TB HDD. I used the existing repair instructions to replace this with a WDC Green 4TB HDD - exactly the same except double the capacity. When the Time Capsule restarts you get an error message in Airport Utility saying the "Disk needs repair". Click the error message, then hit "Erase" and Airport Utility formats a 4TB partition on the new HDD.