my hard drive crashed and i took it to apple and they said it was dead. i got a new Western Digital 500Gb hard drive and tried to install it myself but my computer could not see the hard drive in disc utility etc. so i plugged it in externally using a SATA enclosure. i was able to successfully install mac osx onto my new hard drive and boot up my computer using the hard drive externally but as soon as i put the hard drive back into my computer, it is un able to boot up. I can see the hard drive when i turn on the computer and hold option but when i click on the hard drive it fails to boot up. any suggestions on how i could fix this problem?