SMART HDD is a malicious program for Microsoft Windows that attempts to confuse computer users into thinking it is the similarly named S.M.A.R.T. hardware monitoring system.
It interacts with the user by appearing as a popup stating that the hard drive is failing, and that a subscription is required to fix the issue. Like "Fake-AV" software, SMART HDD will attempt to prevent the user from accessing any applications and will attempt to disable any antivirus software that may be present.
References
SMART HDD Wikipedia(Text) CC BY-SA