Weird problem with a macbook pro won't boot up after putting bottom screws

I'm kind new in this world of apple, and I have a intel's mac that suddently fail the hard drive, so I make a MAC OSX USB, and realize that my old Hard drive was died (Disk utility didn't detect it). I Bought a new HDD (500GB Toshiba) I format and Initialize the hard drive in a PC (because without that the MAC OSX Installation doesn't recognize it) and for the first time i putted all the screws like mint installation, for my surprice the HDD was not recognized, get it out and checked in a PC, the HDD still working perfect, reformat and re initializes the GTP.

For the second time just for test i leave the screws and strech the flex to the outside of the mac (for mantain the hard drive in the outside) and boot the Installation USB... Works!, I installed the OSX and everithing fine until I putted everything in place (Hard drive screws, bottom cover and screws etc) turned on MAC and Hangs on Apple Logo, I restared it, and then the circle with a slash. Booted from usb and Disk Utility won't format the MAC OSX partition with an "Couldn't be unmounted" error.

For the tirth time I reformated the hard drive, reinitialized the GTP with the pc, I putted the hard drive Inside with its screws and this time put the bottom cover but without screws, everything works perfect, after installation navigate on internet login on Store etc, everything good. I restared several times to make sure its booting correctly, and it did. So I flipped over to put the bottom cover screws (Mac still ON), I putted just two of them (just for testing) one of the long in te back middle, and the short one in middle front. When I was in the middle of turned it over I clearly heard the HDD was sudendly turned off (like when you push the power button for several seconds), but the mac still on and apparently working, I restared and then hangs on Apple's Logo. Maybe I wrong about the screws

Anyone have and idea what could be making this fail? Any clue is welcome