… vielleicht hilft Dir das weiter. Beim Start des ersten Backups solltest Du eigentlich gefragt werden, im Notfall kann man es wohl per Terminal forcieren.
»If you used Migration Assistant to set up the new Mac, at the first backup Time Machine may ask if you want to inherit the backup history of the old computer. Note that even if you choose to inherit the backup, Time Machine cannot pick up where it left off and copy only new files. It always copies everything from the new Mac to the backup. Even so, inheriting can have benefits.

Inheriting a backup allows the user to easily restore files from the previous computer’s backup even if they weren’t on the computer when you set up the new Mac. And if the backup drive eventually fills up, Time Machine will seamlessly remove the old Mac’s backups to make room.«
http://www.creativetechsupport.com/help/it/install/timemachine-inherit.html