Why can't I boot from my hard drive internally but no problem externally?

Being unable to 100% solve my problem based off solved problems I have decided to log in. I will start from the very beginning. The computer is the Macbook late 2009 unibody. I upgraded the ram from 2GB to 8GB. everything was going smoothly until like 2 months ago. My 250Gb Hard drive was geting maxed out and in the midst of downloading I was getting warnings to free space so I was deleting and downloading the same time.....and then boom...the computer froze. Couldn't forse quit or anthing only choice was to manually power off. when I powered back on the screen was grey and would not start. I reserched solutions and ended up resetting the PR ram. after that the screen changed to white and nothing happened for a long time I was stuck...until......:

I bought an external hard drive incasing. and like magic everything I thought I lost was right there booting externally. Now I'd be fine with that if not for the reason I bought the computer in the first place, which is to be ultra portable and kep me awake as I work nights as a security in various changing post on a nightly basis. lugging the external case around has complicated a system that was working fine to the point of death.

So over time I have acumilated these things:

a 16 GB hp flash usb currently holding the bootable snow leapard DVD installer

a 500GB wd scorpio blue hard drive from an old PC pased laptop

a 250GB toshiba hard drive (came with laptop snow leopard installed)

a 250GB can't think of the brand cause I'm using it now hard drive (mountin lion installed)

neither Hardive holding apple operating systems will boot internally but boot externally. I have tried putting snow leopard, lion and mountin lion on the scorpio blue but everytime I get the same result, an install failed error. So you may have guessed, I was hoping the scorpio blue as my last hope, getting an operating system on it maybe I could boot internally but for what ever reason it won't intall fully any mac os X. So at this point I want to know if it's possible for me to just put a windows operating system on it ad if it may be able to boot that way.

EXTRA INFO:

when partioning the hard drive, if I have it connected internally, the computer recognizes it but I get cannot unmount errors. externally everything moves smoothly but it won't install the operating system. Currently I'm running mountin lion externally and I have the big hard drive inside the computer and it shows up as "untitled 1" it contents is two folders "Mac os X Install data" (red icon attached) and "private" It tells me I don't have permission to look inside the mac os folder. if I drag a fle from my computer to untitled 1 it says "The operation can’t be completed because an unexpected error occurred (error code -50)."

HELP! whatever it takes I just want to boot something without lugging this external case everywhere.