S.M.A.R.T. System drives are disk drives equipped with Self-Monitoring, Analysis, and Reporting Technology. When used in conjunction with S.M.A.R.T. System software such as Norton Smart Doctor, S.M.A.R.T. System drives are able to alert you that a drive is likely to physically fail before the actual failure. The S.M.A.R.T. System helps to protect your data by allowing you time to back up and save the information on your drive before it becomes unstable. S.M.A.R.T. capable drives are available from most drive manufacturers.
Even though your system has one or more non-S.M.A.R.T. System drives, the Norton Smart Doctor 'Drive Space' gauge is functional and you may continue to use this software if you want. However, remember that on non-S.M.A.R.T System drives failure prediction alerts are not reported.