Time Machine backup not appearing when booting with Option key pressed

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?

Check the link below for the steps.


My drive is supported.

By trial and error it turns out that the drive needs to be turned off when the option key is pressed. Turning the drive on then shows the drive as an option