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

So my 4 month old toshiba laptop's speakers got defect and I took it back to best buy geek squad where it was sent off to geek squad headquarters in atlanta where they replaced the speakers. They also said they changed my laptop's hard drive. They said they changed it verbally and on paperwork they gave me. Since it was under warranty i didn't have to pay anything. I'm amazed at how they changed it without me losing my data, it looks just like when i left it there. How were they able to do that ?? It's amazing how they did that.