SSD w/ brand new clean in stall of Yosemite fails to boot

I've just installed a new version of Yosemite on a third party SSD (Crucial). When I boot it up it hangs on a gray screen.

Steps that got me here: 1) Connect SSD to Macbook Pro (w/ existing HD) using usb 2) Install Yosemite onto SSD while connected to MBP 3) Complete installation -> at this point, I'm up and running with a working Yosemite running on the external SSD 4) Install SSD into internal port 5) Boot MBP 6) Immediately hangs on gray screen

I've read about issues with Trim and Yosemite, but to my knowledge I've never enabled or installed Trim on this SSD.

The Option key and Command-R are both non-responsive during startup.