USB External Hard drive unplugged but still shows up!

I am running on Ubuntu 12.04. I have physically unplugged an external hard drive from my laptop, but for some reason it is still shown as mounted in Nautilus. When trying to get rid of it by unmounting there is a message that says

"umount: /media/My Passport is not in the fstab (and you are not root)"

When replugging in the same external hard drive, the message says:

"Error: Error when getting information for file '/media/My Passport': Input/output error
Please select another viewer and try again."

Can someone please help me with solving this problem. I am a new user of Ubuntu and I have no background whatsoever, I would really appreciate it if the instructions are easy to understand. Thanks!!

Try running the unmount command with root permissions from the CLI.

Open a Terminal window with alt+t or by searching for "Terminal" in Finder.

Going off the data you provided about the error, it looks like your HDD is mounted at '/media/My Passport'. To unmount it, run sudo umount "/media/My Passport". Enter your password when prompted.

If it is not mounted at /media/My Passport, type df and hit enter. A list of the mounted volumes on your system will be displayed. Find the one for your HDD (will likely say something about "My Passport", and the size will be what you expect). Note the last column of that output for your HDD (labeled "Mounted on"). Let's call it /path/to/hdd for this example. Run sudo umount "/path/to/hdd" to unmount your HDD as root.

I suspect that the second part of your question, regarding plugging it back in, will resolve itself once you unmount your drive properly.