You should never have files stored on only one hard drive if they are very important to you. You should have at least TWO external hard drives with the same data on them. Why? Because all hard drives will fail eventually, and often do so without any warning (especially external ones).
Test the drive with Seagate SeaTools for Windows
. Click "Downloads" on this page: http://www.seagate.com/support/downloads/seatools/
If the drive passes the test and you need further advice on this problem, please post again, and mention that the drive passed the SeaTools test.