Q: Internal SSD Not Recognised

Install a new internal SATA cable.  A search of these forums will develop many users with the same issue:  HDD + SSD work externally when connected via USB.  HDD works internally, but the SSD does not work internally.

Ciao.

Do an Upgrade of RAM and SSD for Best performance.

I have the same Macbook Pro (MBP) Mid-2012 13-inch, Non-retina. I did an SSD and Ram( Memory ) upgrade.

Since you have the MBP open you might as well upgrade booth SSD and RAM.

I suggest a 500gb. SSD and 16gb of ram. Just max out your MBP for best performance it will costs you about $200

However if you want to do a minimal cost upgrade I suggest a 240gb. SSD and 8gb of ram, It will cost about $100

I used a 500gb. SSD beacuse I used bootcamp app and created two 250gb partitions for dual boot of OS-X and MS-Windows-10.

You will be very happy after the upgrades, I promise. My MBP just flies threw any task I have it perform. It works great.

This is how the MBP should have came NEW from Apple.

For best performance SSD is more important than Memory

El Capitan and apps requires a minimum of 8gb. ram for good performace. 16gb. is better

Here is a list of parts

SSD - OCZ trion-150-series SSD

https://www.amazon.com/Toshiba-OCZ-Trion-Internal-TRN150-25SAT3-240G/dp/B01B4NUK MY/ref=sr_1_1?ie=UTF8&qid=1470689650&sr=8-1&keywords=ocz+trion+150

Or Samsung Evo 850-series SSD

https://www.amazon.com/Samsung-850-EVO-2-5-Inch-MZ-75E250B/dp/B00OAJ412U/ref=sr_ 1_1?s=pc&ie=UTF8&qid=1470691960&sr=1-1&keywords=samsung+evo+850

Or OWC SSD

https://eshop.macsales.com/shop/ssd/owc/macbook-pro/2012

MBP Mid-2012 RAM ( Memory )

8gb.

https://www.amazon.com/Crucial-PC3-12800-204-Pin-CT2K4G3S160BM-CT2C4G3S160BM/dp/ B008LTBITY/ref=sr_1_1?ie=UTF8&qid=1470689595&sr=8-1&keywords=8gb+mac+memory

16gb.

https://www.amazon.com/Crucial-PC3L-12800-SODIMM-204-Pin-Memory/dp/B007B5S52C/re f=sr_1_3?ie=UTF8&qid=1470689595&sr=8-3&kÖ

Here are some how to video for do-it-yourselfers

https://eshop.macsales.com/installvideos/macbookpro_13_unibody_mid12/

To clone your hard-drive to SSD I used "carbon copy cloner app" ( Just down load and install, run as trial for FREE use ).

https://bombich.com/

You will also need a USB to Sata adapter

https://www.amazon.com/StarTech-SATA-Drive-Adapter-Cable/dp/B00HJZJI84/ref=sr_1_ 3?ie=UTF8&qid=1470690166&sr=8-3&keywords=usb+to+sata+adapter

And a torx-6 screw drive, small phillips and flat head screw driver

https://www.amazon.com/JACKLY-Professional-Screwdriver-Extension-Maintenance/dp/ B005JRTAYG/ref=sr_1_4?s=hi&ie=UTF8&qid=1470690234&sr=1-4&keywords=torx+screwdri v er+set

After you clone your drive to SSD and get it installed you will need to enable "trim" support for your NON OWC SSD

Open "terminal app" and enter this command "sudo trimforce enable" Select YES, YES and reboot when complete.

Also remember to reset SMC,PRAM, and NVRAM on your MBP

http://www.macworld.com/article/2881177/how-to-reset-your-macs-nvram-pram-and-sm c.html

How to reset NVRAM (or PRAM)

If your Mac is not booting correctly or one of the functions listed in the previous section is acting strangely, you can perform a simple reset of your Macís NVRAM or PRAM. Resetting your Macís NVRAM or PRAM will reset the data stored to its default setting. (Note: As always, we strongly recommend having a complete backup of your important data before troubleshooting your Mac.)

The process for resetting PRAM and NVRAM is exactly the same. Simply follow these steps:

1) Turn your Mac off.

2) Turn on your Mac and when you hear the startup chime, immediately press and hold the Command-Option-P-R keys. (Note: You may need a wired keyboard for the reset).

3) Hold the keys until the Mac restarts and you hear the startup chime for a second time and then let go of the keys.

After resetting NVRAM or PRAM you may need to reconfigure some of your Macís settings. If you are still having issues with your Mac, you might want to consider resetting its SMC.

What is SMC?

On Intel-based Macs, SMC stands for System Management Controller. The SMC chip controls your Macís sleep function, power, various lights and indicators, fan speed, and keyboard backlights among other things. So if your Macís fan is out of whack, indicator lights are misbehaving, it wonít respond properly when closing the lid, or the Mac wonít turn on at all, you might want to reset the SMC. (Note: Again, be sure to have a complete backup of your important data before resetting the SMC)

How to reset SMC in a MacBook with a non-removable battery

1) Shut down the Mac.

2) Connect your power adapter to your Mac and to a power source.

3) With the Mac powered off, press Shift-Control-Option and press the power button at the same time.

4) Release all keys and turn on your Mac.

Your total cost might be under $200 however it will be well worth it. Your MBP will perform like a NEW $2000 MBP

You will be very happy. It might take you about 1 hour to perform this upgrade. Mainly because the cloning process takes a long time. The actual hardware upgrade will only take you about 15 minutes to complete.

Your total cost might be under $200

Or it might not, and it seems like a lot of work / expense for a cable issue.

Very commonplace for that cable to fail when a much faster drive is connected, fortunately it's easy to fix.