Refusal to Boot if NON-BOOT internal HD is plugged in

Aug 11, 2013
i7 3770
geforce 670
Classic Mac
Mobile Phone
My new hackintosh has been working fantastically for about a month. It boots from an internal SSD (where chameleon and ML are stored) but also has my old macbook 2.5 HD installed with a mounting bracket. The old 2.5 has been reformatted to exFat and was basically my dropoff point for downloads, music, etc.

Everything was working fine until I told Steam to change its 'steamapps' folder to this 2.5 HD. I told it to dl/install a game and when I came home later, the harddrive was corrupted. Not broken, mind you-- it still showed up in disk utility-- but unmountable. I should have reformatted it right then, but I wanted to save some files so I used a few freeware tools (testdisk and some scanning software) on it. No success. I think I may have buggered it even further with testdisk, as I attempted to manually re-enter the block information for each partition, following this guide:

Here's where it gets weird.

Now my computer won't boot AT ALL so long as the HD is plugged in. Chameleon freezes before it can begin loading the OS. I've told my BIOS again and again to boot from my SSD and NOT from this wacky 2.5 HD, yet it still freezes so long as its SATA cable is plugged in. I've tried waiting for chameleon to begin loading properly, then jacking the cable in, but the OS can't find the drive at all.

I want to reformat this drive, not throw it away, but can't find any way to run disk utility on it! Any advice?

EDIT: My brother had a SATA --> USB enclosure lying around, so I was able to reformat using that. I stuck with Mac OS Journaled this time, just to be safe. If anyone has speculations about what might have caused this corruption in the first place, that'd also be awesome. Is exFat just unreliable for an internal drive that constantly talks with ML? Is a program like Steam liable to get confused when writing an application with a back-end in a non-root directory?