HDD
From Hackepedia
HDD stands for "hard disk drive", typically where data is stored on computer. As this is a mechanical device, it is one of the more likely devices to fail inside of your computer. For this reason, make SURE you do a backup of any important data you have. If you do think your HDD is dying, or are having issues and would like to check, all major HDD vendors offer a diagnostic boot cd image. This means you should download it, and burn it onto a cd, and boot your computer off it, which will step you through the process of testing your HDD for issues.