Comment 8 for bug 292493

I had the same problem using UNetBootin to create the USB device. I thought that I could tell it not to edit the bootloader and then I would edit it later using a separate install of Ubuntu however the problem persisted.

The problem appears to be with the installer it self. I opened up a terminal to try to manually force unmount the /cdrom mount point it failed even when done as root. It continued to insist that the partition was in use and could not be unmounted.