Dave I think you facing the optical drives SATA port issue many of the newer MacBook Pro's have.
Basically the Optical ports SATA port is not the same speed as the HD SATA port.
You'll need to get a fixed SATA speed drive SATA II (3.0 Gb/s) for use in the optical drive. Review this OWC note in red: OWC Data Doubler.
The Seagate SSHD drive won't work as the auto SATA speed sense circuit can't handle the odd SATA speed the optical drive SATA port in the system has.
I would recommend you move the Seagate SSHD drive over to the HD location and depending on what your HD is you might be able to move it over to the optical drives SATA port.
Just to be clear here, the issue is not the carrier its the Intel controller chip Apple used and the given CPU's speed (GHz). So no matter who's optical HD carrier you get they all have this problem.
I would also make sure your firmware is up to date. Review this Apple TN on how th check and update: EFI and SMC firmware updates for Intel-based Macs
One way to verify what you have is to see what the System Profile report tells you. To access it go to the Apple menu > About This Mac > More Info button > System Report button, then slide down to Storage. Then going to SATA/SATA Express will show you the different volumes. Take a snapshot of the two and post them here for us to see. If this is a WD Black² HD you should see two disks.
What happens when you use an external case or SATA to USB cable adapter to host the hard drive. Are things are faster?