Instructions
Inspect the hard drive on the side of the Xbox 360. Make sure the hard drive is connected firmly and properly to the console. The drive will appear level inside the hard-drive bay if the device is correctly inserted.
Power off the Xbox 360 console and remove the hard drive by pressing the button on the side of the console and pulling the drive outward. Turn the console back on and power it off again to refresh the console. Reattach the hard drive by pressing the large end into the back end of the drive bay and press the front end into the bay until it clicks into place.
Power the console back on. Highlight "System Settings" from the Xbox 360 main menu and press the "A" button. Highlight "Memory" and press "A." The hard drive should appear as a source of memory if the device is functioning properly.
In the event that the hard drive is still not functioning properly, remove the hard drive and test it on another Xbox 360 console, if possible. If the hard drive still will not work, contact Xbox Customer Support (see Resource).