My Book Studio for Mac USB 3.0 not bootable

I have an Intel Macbook Pro Retina 15" and the drive is formatted to GUID.  I bought the silver Mac edition, but I also tried reformatting to with GUID.  It sitll does not show up under Startup Manager.  However, it does allow me to select it as a startup disk in System Preferences, and that works fine.  Any ideas?