internal HD no recognized hooked up externally its all good. why???

so I'm a dumbAzz I just bought a new MBP so decided it was time to finally take a part my 2008 MB and fix the dvd drive. well I unscrewed everything pulled the battery, popped the ram outta both slots pulled the HD then the keyboard and finally the dvd drive I fixed the dvd drive reassembled and bam i get the question mark screen (insert curse words here)
I pulled the HD hooked it into an external enclosure and it works.
now I reinstall HD reset my pram start up w/ C while install disc is in go to disc utilities no HD to be found (more curses) pull the HD do the same thin with it hooked externally through the USB it is now seen in the install set up and I'm able to reinstall
Any ideas? I just want it back together in proper working order so I can sell it and the Ipod i got for the education purchase and buy an ipad.

Thanks

Pull the hard drive once more from your MacBook; now check in the slot where the hard drive slides in - use a flashlight. Look on the sides of the slot to see if the rail insulators are in place. Sometimes they can slide out and interfere with the connection. There should be two rails; one on the left, the other on the right. If either is loose or obstructing, push them back in place and carefully slide the drive in so as not to disturb them.

Regards.

thanks I'll take a look at that