My external hard drive is not showing up on my window 7 desk top?

I have a 600gb WD hard drive. When i plug it in the light are on, but when i go to computer, it doesnt show up. I go to device manager and it has i yellow thing on it and it say windows uninstalling, i uninstall it and unplug it and restart the computer and plug it back in it does the same thing. help me please i have a lot of things in there