How to fix: external hdd not accessible

How to fix: external hdd not accessible

The WD-My Book 500GB is recognized as a local disc in my system (Windows 7), but it's drivers fail to install successfully when connected via usb cable with computer. Then when I try to open it (explore), it slows down the pc, and after some minutes it displayes the message: 'F:\ is not accessible'. And after some time it shows another message asking me to format it. I really do have important data i wanna get from it, so i can't format it.