The Xbox 360 uses as a storage option an external hard drive, mounted to a carriage on the side of the console. The hard drive has increased storage capacity over a standard memory card, making it ideal for storing games, saves and content purchased from the Xbox Live Marketplace. Should the hard drive malfunction, and the cause isn't physical damage, there are a few techniques to get the console to recognise the drive again.
Ensure the drive is fully seated around the perimeter of the unit. Press the small button toward the front of the drive to remove it.
Lay the rear of the drive into the corresponding section of the drive's cradle. Lay the unit straight down. Press gently on the top of the drive to click it into position.
Turn on the console. If you are sure the drive is fully mounted, and the console still fails to recognise it, clear the internal cache. Do so by heading to the "System" page via the left stick on the controller. Press "A" on the controller.
Press the "A" button on the controller to get into the "Memory" submenu.
Press the yellow "Y" button to access the hard drive's options.
Press the following sequence on the console's controller: "X," "X," "LB," "RB," "X" and "X."
Cycle up to the "Yes" option via the controller when the console asks if you wish to clear the cache, and press "A" to initiate.
Remove and reinsert the hard drive once the clearing function is completed. Note that all game and system updates are cleared, but you are prompted to download them again anyway once a game is reinserted and/or the console is rebooted. No progress save data is lost.
Never remove the hard drive while the system is writing or accessing data from the drive.