Why does my drive report a S.M.A.R.T error?

Turned my computer on and once Windows started I logged in and got a message that my drive has suffered a S.M.A.R.T error.

S.M.A.R.T errors are self diagnostic’s performed by the drive. It could be that the temperature has gotten too high or that it has found bad sectors. I would suggest you copy anything important to another drive and run HGST Windows Drive Fitness Test (WinDFT) it should give you more info about what errors exactly its reporting.