3.5 inch SATA hard drives

You need to get an external enclosure for a 3.5 inch drive. The enclosure must include a power supply for the drive. Check out OWC for such enclosures. It will need to provide connection via USB port.

I've always used enclosures from OWC:

http://eshop.macsales.com/shop/hard-drives/External-Enclosures/3.5-Inch-Drives

They are always Mac compatible never a problem in 9 years.

So in your last post:

"in response to macis1337"

you're talking to yourself. So it's hard to tell what you're responding to.

When you only supply the model ID it could be any of these:

http://www.everymac.com/ultimate-mac-lookup/?search_keywords=MacBookPro9,2

In addition:

" I used an SSD with custom configuration, and I think it is PCI-E. "

So the interface in the 2012 was a SATA. If you upgraded to an SSD it would have used the SATA connector.

"So I probably have a spare SATA slot"

No that will be taken up by your current SSD.

"The only issue is power. will the mac be able to provide 12V?"

"Also I'd like to connect via Firewire 800"

OK that would be fast but you'd still need an interface. How are you going to make the transformation between SATA and FireWire 800?

To do that will require a very specific chipset.

Finally where will the 12 V @ 1.73 Amps peak come from?

"I don't really need my Optical Drive, so..."

So for that you're thinking of something like Data Doubler:

http://eshop.macsales.com/item/OWC/DDAMBS0GB/

But:

"Testing has demonstrated that Apple factory hardware does not reliably support a 6G (6Gb/s) Solid State Drive or Hard Disk Drive in the optical bay of 2011 MacBook Pros (ModeI ID8,1; 8,2; 8,3). If your OWC Data Doubler bundle comes with a 6G drive, you should ONLY install that drive in the main drive bay and utilize the Data Doubler to re-task your existing drive or install a new 3G SSD or HDD in the optical bay. PRE-2011 models can utilize a 6G drive in the optical bay, but will do so at a reduced 3G (3Gb/s) speed."

In addition:

" This custom engineered blue anodized aluminum bracket with attached PCB circuit board comes ready to mount any 9.5mm tall 2.5" SATA hard disk drive and even Solid State Drives"

So yours is 3.5" and will not fit anyway.

""The only issue is power. will the mac be able to provide 12V?"

No, not at that power level.

"The Problem is Power."

No the problem is size.

"Do I need an External Supply"

Yes.

"can the Mac provide the power?"

No.

"If I had to go with an external case and I can't use SATA,"

The external case would have a SATA connector in it.

"I'd like to connect via Firewire 800."

Yes the page I reference would have FireWire 800 connectors on it. Call and make sure.