1520 HD... what happened?

So ... I received a replacement 1520 system for the original with the bad screen. Only thing is, the HD in the new one makes this annoying loud random clicking sound (not connected with any lights going on or off).
I tried adjusting the acoustics in BIOS, as I read in another post on this forum, but that didn't help.
Having read about someone that just switched the new HD with the original, I thought I'd ask if it was ok and contacted first:
Customer Care chat: who informed me that the Tech dept would be better able to answer that question.
Then Technical Support chat: who informed me that switching the hard drives would not be practical, and suggested that I contact Customer Care.
Then I called Technical Support, not wanting to get caught in the loop AGAIN... and this person, after confirming that I wanted to exchange the HD for identical systems, said fine... and waited for me to get a screwdriver after determining that I had done this before exactly once and could use the help.
Long story shorter, I couldn't get the screws out with the screwdriver I had, so rang off after he walked me through the very simple process and told me to call back if I had any problems.
I bought a different screwdriver, swapped the drives, and booted up the replacement... no problem.
I booted up the original, which now has the clicky drive, and Windows couldn't start, even after it went through its automatic trouble-shooting process.
Anybody know what might have happened?
I presume there is no problem with returning it in its current state, but I'm confused!

 

The clicking sound could very well be an indication of a mechanically faulty drive. If I understand correctly, that's the one your returning, so I wouldn't worry about it.

 

whew... if that's the case ~ that it was a faulty drive, I'm happy I switched them!
Thanks!