Seagate makes two free software programs for testing hard drives: SeaTools Bootable and SeaTools for Windows/Linux. While both of these testing tools are excellent, they are different.
Seagate Seatools Diagnostics | Run diagnostic tests on your hard drive
The bootable version of SeaTools is more powerful, but is a bit more difficult to use. SeaTools for Windows or Linux is not as full featured, but is much easier to install.
Both of these programs are completely free and highly recommended. If you suspect your hard drive is bad, one or both of these Seagate tools can help you determine what is wrong.
Seagate SeaTools is available in both a Windows (or Linux) version and a standalone, bootable version for maximum flexibility, called SeaTools for Windows/Linux and SeaTools Bootable respectively.
