Why can't I see my hard drive? I'm running El Capitan on Mac OS X.

MacBook would not boot (would hang on the Apple logo progress bar screen during start up).

I brought it to Apple Store genius bar, guy said the hard drive is still functional but there was a warning on it so I should buy a new hard drive.

I bought a new hard drive, installed it, installed El Capitan on it.  Everything works.

Then I put the original Mac HD into an external HD USB enclosure.  I plugged it in, and the light flashes on the hard drive enclose, but the Mac (and Disk Utility) cannot see the hard drive.

Anyone know what's wrong with the drive?  I'd like to copy some file off of it.

If it's just "dead" and there's no way to make it work normally again, is there another way to copy the files off the hard drive?

Thanks!
Turns out I had a hard drive failure which is why my Mac couldn't see the drive and why Disk Utility couldn't see the files on the drive.

After trying many things, I found a utility called Data Rescue 4 for Mac and it scanned the drive, found all the files and I was able to recover everything.