You are likely facing two issues here.
The first dependent on your exact model may be facing a compatibility issue with the Optical drive's SATA port. Review the Red comment at the bottom if you have a 'MacBookPro8,1' (2.3 GHz, 2.4 GHz, 2.7 GHz or 2.8 GHz system: OWC Data Doubler it makes no difference who's unit you used they all face the same problem here. As you can see the drive you have installed in the optical carrier needs to be a fixed SATA II (3.0 Gb/s) drive.
The other issue you maybe encountering here is the issue of HD crash guard. You see only the main drive port has it. So if you switch over the original Apple HD to the optical port you won't have any protection! Apple drives don't have the technology onboard the drive (some replacement HD's from 3rd parties do!) So you need to be careful with the banging around when the system is running (or in sleep mode).
OK, lets say you switch things around or put in a different HD in the optical carrier and you need a SATA II drive. You'll need to be careful as 'Auto SATA Port Sense' drives also won't work very well here. Depending on the manufacture the drive could set its self in SATA I (1.5 Gb/s) mode as it can't determine what the ports speed is or it will pop between SATA II (3.0 Gb/s) and SATA III (6.0 Gb/s) very confused! As sometimes it will work and other times not. Its best to get an older FIXED SATA II drive here.