I have two WD 3TB USB3.0 My Books, and TimeMachine is working fine on both of them.
However, when I boot up the machine with the Option key pressed neither drive appears as an option to boot from (the only option I get is “Macintosh HD”). I have tried having one drive with encryption and the other drive without encryption but neither work.
This is a late-2012 iMac with ML. Any ideas?