Even though the Xbox 360 S or “slim” doesn’t have a big hard drive unit attached to the top or side of the console, there is still a hard drive in most models of this latest revision of the Xbox 360. The 4GB model does not have a drive included, but the very popular Xbox 360 250 GB model has a hard drive hidden on the inside. If, for some reason, you need to remove and re-seat the drive or replace it entirely, the steps are quite simple.
You simply need to remove a small plastic cover from the end of the console and pull the plastic tab to remove the drive. The steps, and some easy to understand pictures are on support.xbox.com, as always, and the link also includes how to remove the hard drive on the original Xbox 360 (which is mostly self explanatory). Good luck with it!