I am sorry to post this here, but the stuff via Google search seems unhelpful.
I have a MacBook Air. When I first plugged in an external Seagate USB 3.0 drive back in May, Time Machine knew to automatically back up the whole thing. Great.
Now, whenever I try to plug the Seagate back in and let Time Machine do its magic, it won’t work. Specifically, Time Machine stays stuck in the Off position. I slide the lever to the right, to turn Time Machine on. It then has a window open, asking me to select the Backup Drive. So I pick the Seagate, the window closes, and the Time Machine window slides the slider back to Off (and nothing happens).