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

So my 4 month old toshiba laptop speakers got busted so i took it back to best buy geek squad and they sent it off to the geek squad head quarters where they replaced my speakers. They also said they changed my laptop's hard drive too. So when i picked it up they said the speakers was replaced and so is my hard drive, they said it verbally and it was on paper. I'm actually amazed at how they changed my laptop's hard drive without me losing my data. How were they able to do that ??