MBP does not recognizes the additional HD

Yes, it does sound like the optical drives SATA cable is bad here or the ports logic.

You may want to review OWC: Data doubler page. While you have someone else's unit they all have the same problem due to the system controller chip Apple used. In your case you have a MacBook Pro 15" 2.53 GHz Core 2 Duo {MacBookPro5,4}. Here is what OWC states:

2008/2009 MacBook/MacBook Pro 13", 15", and 17" models. (Model IDs: MacBookPro5,1; 5,2; 5,3; 5,4; 5,5 and MacBook5,1; 6,1; 7,1) While a 6G SSD does function, it will only do so at SATA Revision 1.0 (1.5Gb/s) speeds rather that the SATA Revision 2.0 (3.0Gb/s) speed the machine can deliver. Should owners of these machines desire another SSD option, the Mercury Electra™ 3G SSD does run at the full SATA Revision 2.0 (3Gb/s) specification.

Basically your Optical drives SATA port is only SATA I (1.5 Gb/s) in this model. So you are limited to drives which are compatible with SATA I. The original Apple drive is a SATA II (3.0 Gb/s) and may have problems running here in the optical drives SATA connection. I should also point out the crash guard protection Apple has in their systems is only within the HD SATA connection. So moving the drive over you loose this protection.

Frankly, I don't recommend doing dual drives in this system given all of the issues here. I would recommend going with a SSHD hybrid drive Seagate laptop SSHD replacing the HD alone. Then you gain the performance of the SSD and still have the deep storage of a traditional HD. Or, if you can swing it get a larger SSD alone and be done with it.