|The most common cause of that happening is improper removal of the USB device. You should always use the safely remove icon in the tray by the clock to remove USB devices.|
You may be able to repair the damaged partition by using a program called testdisk. Be sure to read all instruction before using. Get testdisk at the link below.
In the future I suggest you keep at least two copies of your important personal files. Try burning them to optical disk in addition to storing on the external drive.
I removed your second post on this problem. Double posting is not allowed.