Why doesn't my external hard drive appear in my MacBook Pro?

Why doesn't my external hard drive appear in my MacBook Pro?

I checked if it's hidden but it's not. It's not in the sidebar too. My Samsung G2 Portable external hard drive lights up and starts with a typical sound when you plug an external hard drive to a computer. If I can remember I didn't drop it or anything but I think my macbook pro automatically forced eject it. I tried plugging it on different macbook/iMacs or even PC computer.