It's weird. It works so well for me that I don't get why others have a problem. I also don't understand why Apple yanked it as a published feature when they used to. Here is how I got it to work. Well, both ways I got it to work.
Way 1
A) Follow the directions to setup a disk to be accessed while connected to the AEBS. Should be in the help.
B) Mount up the disk in Finder.
C) Go to Time Machine prefs and set it up like you normally would and select the disk as the backup destination. Don't mess with the "Setup Time Capsule" stuff.
From there it should backup whether you have the disk mounted or not. If you don't it will mount it for you then unmount it when it's done.
Way 2 - not too different from Way 1
A) Plug the USB drive into your computer and setup Time Machine to use it. Run a backup.
B) Follow the directions to setup a disk to be accessed while connected to the AEBS. Should be in the help.
C) Mount up the disk in Finder and run a backup.
Again, from there it should backup whether you have the disk mounted or not. If you don't it will mount it for you then unmount it when it's done.
That's how it has worked for me. I have two Mac's backup up this way no problems. Feel free to private message me if you need some more help.
