If Disk Utilities will not see it, there's not much you can do. Open it up and see if possibly there's a jumper on the drive. If so pull the jumper. Next I would remove the drive and place it in the external enclosure, start up from my system installation disk and use Disk Utilities to format it and install a system. If you can't see it in the external, return it. To test if you got everything back together correctly, insert the old hard drive.