You maybe hitting a known issue with this series. You talk about adding a second drive so you replaced the optical drive for the second drive using a product like this: OWC Data Doubler. It makes no difference who's you used as they all face the same problem here. From the OWC Data Double notes:
- MacBook Pro 13" models: Apple does not support the use of 6Gb/s drives in the optical bay. While we have observed a high rate of success using SATA 3.0 6Gb/s drives in Apple 13" bays where 6Gb/s link is present, some systems may not operate properly with this setup. For guaranteed reliability/compatibility, we suggest 6Gb/s drives be used in the main drive bay only, and 3Gb/s hard drives or SSDs be used in the optical bay when a two-drive configuration is desired. We cannot guarantee proper or successful 6Gb/s drive operation in the Apple MacBook Pro 13" optical bay.
You see the problem here is the optical drives SATA ports data rate is not a clean SATA III (6.0 Gb/s). So you need to use a FIXED SATA II (3.0 Gb/s) drive. The Seagate drive you have if I understand what you got is this: Seagate Laptop SSHD as it turns out this drive won't work either as unlike a fixed SATA II drive this drive has SATA auto sense technology so it is able to match the systems SATA port. In this case it's not able to correctly match the system port speed.
So what to do here??
You'll need to hookup the drive using something like this externally to get around the systems SATA port issue: Startech 2.5" SATA to USB adapter. Then you'll need to try to using a data recovery application like this: Data Rescue
If this is unable to gain access to the drive you'll need to talk to Seagate to see what they can do for you.
First I agree with @mayer here replace the internal HD drive cable as I suspect it might have been the root cause of your problems with this drive! You don't want to use any drive in the bay until you do. MacBook Pro 13" Unibody Mid 2012 Hard Drive Cable Replacement & MacBook Pro 13" Unibody (Mid 2012) Hard Drive Cable
If you can get your self to an Apple Store or authorized Apple service center they will replaced it for free. You will need to have a drive in the bay though so think which drive you have on your shelf you can use. Do explain to them the SSD is the boot drive.
So at this point the SSHD drive just won't run when connected externally. At this point I don't think you have any alternatives here you'll need to sendoff the drive to a data recovery service. There they can use some special tools to recover your data from the drive. But that will be expensive! Are you sure you have no backups or have any way to recover from other sources the info that was one the drive? You might want to try that first.
You could try contacting the maker of the drive to see if they offer any guidance on how to proceed. Otherwise it sounds like the drive is gone here. Sorry ;-{