HDTune is a free tool that works on all types of hard drives, even external ones, USB sticks and iPods:
It comprises the following features:
-Benchmark: measures the performance
-Info: shows detailed information
-Health: checks the health status by using SMART
-Error Scan: scans the surface for errors
I think it is the best "first stop" for troubleshooting. Apart from scanning and status checking it also will identify the drive, which is what makes is useful for people who don't really know much about computer hardware.