After doing further research, don't think that either of my suggestions will work. Lets try this: Drag the EFI Disk (or whatever it's named) which is currently on the Desktop into Terminal. It should appear as something beginning "/Volumes/..."
Select and copy that, Cmd-C, quit Terminal, re-open and then enter the following
sudo unmount /dev/disk0s1 /Volumes/efi
Important, replace "/Volumes/efi" in the above with what you copied earlier--use Cmd-V. Make certain to preserve the single space between /dev/disk0s1 and /Volumes/efi, or whatever its name was that you copied from Terminal.)
Next, you will be given a scary warning about using sudo. Ignore that and keep going and when prompted, enter your admin password and hit Return. Note, when typed in, you won't see your password in any form, not even as *****.
Wait until you get a new $ prompt in Terminal, which will mean the command finished, then close Terminal. If it responds with some kind of error, please copy and post that here.