Self
Monitoring
Analysis and
Reporting
Technology
Built into the hard drive it measures such things as:
Spin up time, head hight above the platter, data throughput etc.
It then uses these bits of data to predict when a drive is on its way out and if so puts a message on your screen warning the disc is a bit dodgy. You should then have enough time to get the data off before complete failure.
I usually leave SMART on, can affect data throughput slightly but what the heck - I'd rather know when I'm likely to be needing a new disc.