Recently purchased CC121. I have loaded, installed, reinstalled drivers and extension (from Steinberg.net), also from Yamaha for the Yamaha USB MIDI Driver as well, reinstalled again from Steinberg.net, tried different USB cables, updated CC121 software to be sure (even thought the lights indicated it was already current). The CC121 shows up in the Cubase Application Device Setup, Windows Media I/I In and indicates Active. I start the CC121 first then launch Cubase. CC121 just blinks blue…and I am getting red with frustration. Welcome thoughts and ideas. Left message with Steinberg under My Support but haven’t heard back yet. HELP PLEASE!
That’s annoying 
What version of Windows are you using?
I am using the following
Windows 10 Pro. 64-bit, 8GB RAM
i5-4300U CPU @1.9Ghz
Have even tried reinstalling Cubase Elements 8. Still a blue blinking light.
Problem solved. The CC121 does not work with Cubase LE. Which is the version that apparently I was running. When I switched to either AI Elements or higher all works great.
Special thanks to Josh at Steinberg for working with me and responding so quickly.
Ah, so you lied to us 
Glad you got it working! 