Time Machine seems to think my MacBook Pro is a new backup after Yosemite upgrade
You may want to check this thread: https://discussions.apple.com/message/27031297

It seems that if you backup external drives with Time Machine and you disconnect them at a later point, for instance for a few hours or days, and reconnect them, Time Machine will want to make a full backup of them (even if nothing has changed). That behavior appeared with Yosemite. I have not heard about any solution or workaround (yet).