My mom bought a used iMac, but the person that owned it before didn't erase any of their data. I noticed that it had two HDs on the desktop: the Macintosh HD and some other HD. I got the disks from them today to restore the computer, but I still see both of them on the desktop. The other HD has nothing on it. It is 5.7 GB in size and it's all free space. I can eject it, but it will appear again when I restart the computer. Any idea what this is and why it keeps appearing?