SSD not recognized on MacBook Pro mid 2012

While your system does offer a SATA III (6.0 Gb/s) HD port the optical drive port is not able to support SATA III devices reliably. Review this: OWC Data Doubler go to the bottom of the page and review the Red notation and the note just under it.

Sadly, it makes no difference who's optical drive carrier you use the problem is within the main logic board as Apple did not expect people to use SATA III (6.0 Gb/s) devices using this port.

Even though your Kingston SSD is able to run at SATA II (3.0 Gb/s) it still won't work as it's not a Fixed SATA II drive but one that auto senses the SATA ports speed so it can't latch into the correct SATA speed.

I would recommend you either find an older Fixed SATA II SSD or think about just using the HD port alone here.

If you can't find the SSD I would recommend upgrading your HD to a hybrid drive like this one: Seagate laptop SSHD. I would go with the 32 GB NAND model. Or, make the bigger leap to a big SSD like the Samsung 850 EVO replacing your HD with it. Sadly, I would put back the optical drive at least you could play some DVD's with it ;-}

Here's the IFIXIT guide you'll need to follow: MacBook Pro 13" Unibody Mid 2012 Hard Drive Cable Replacement and here is the part you'll need: MacBook Pro 13" Unibody (Mid 2012) Hard Drive Cable

I must warn you! Putting the original Apple HD into the optical drive carrier is not smart as only the HD SATA port has crash guard protection as the drive its self doesn't have it. So you do need to be careful not to bang the system when its on or in sleep mode as you could damage the drive. That why I alway put the SSD in the optical drive carrier so the HD drive gets the most protection it needs.