Added Second Drive - Seen, But Not Working!

Used the iFixIt Mac Mini Dual Drive Kit to add second drive to Mid 2011 MacMini. Second drive is Samsung 850 PRO - 256GB SSD formatted as a bootable drive with Sierra 10.12.2 installed. MacMini now sees the drive but says it can not be mounted, read, erased or formatted. Disk Utility First Aid just gives error message. SSD was tested and working before install. First thought was "bad cable." But, if cable is bad, would the drive show up at all? Help! Ideas? Suggestions?