How did the geek squad agents change my laptop's hard drive without losing my data?

So my 5 month old laptop I bought from best buy got busted and it was a 14 inch toshiba laptop and I took it back to best buy a week ago to have it repaired. It was then shipped to the geek squad headquarters in Atlanta where they replaced my speakers. They also mentioned on paper and verbally when i picked it up that they replaced my hard drive and since it was under warranty, I didn't have to pay anything but pick it up. I'm amazed at how they replaced my old hard drive with a new one without losing my data. How were they able to do that ????