Unable to reformat 2012 iMac - Disk1 is unmountable

Update - Problem fixed!

Hi, everyone! I browsed the forums to try and make sure, but if this is the wrong place to post this, please feel free to move it/have me post elsewhere!

I purchased a barely used 500gb 2012 iMac today from my uncle's neighbor. She is an elderly woman who used the computer to store about 300 pictures and read her emails over the last few years. The computer works great! However, when I go to reformat, I am given the following error:

Disk Erase Failed
Disk Erased failed eith the error:
Couldn't unmount disk.

With that in mind, I began to research how to dismount this disk. Nothing is mounted as far as I can tell. I turned off TimeMachine so that did not interfere, but I am still having the same issue. I attempted several ejections in Terminal and all failed. Here's an Imgur album with three pictures to show you what I'm seeing and what commands I've tried so far:

http://imgur.com/a/j6q33

So what's the issue? What am I doing wrong here? The computer works really well otherwise - I just cant reformat it so I can make it my own. Am I going to be stuck on this lady's profile for the duration of using this iMac? Thanks so much for your help!!