I have to repair my external hard drive each time I plug it? I have a 2009 320GB Toshiba Hard Drive, and every time I plug it to my macbook pro 13inch Retina, I have to go to Disk Utility -> Repair Disk.

Why is that? Whats the solution? Please help!!! Thanks
Try ejecting the external drive before you unplug it. You can either use the Eject icon or use Command + E when its selected int he Finder as mentioned by Tetsujin.