"error unmounting /mnt/source" while installing F8

[Konstantin Svist]

I'm getting an error message while trying to install F8 on my laptop.
When I got to the screen where I select the package options 
(Office&Productivity/Software Development/Web Server), I selected 
"Additional Fedora Software" and chose "Customize now" (so I could 
select packages on the next screen). When I pressed Next, I got the 
following error message repeated ad infinitum: "An error occurred 
unmounting the disc. Please make sure you're not accessing /mnt/source 
from the shell on tty2 and then click OK to retry"

I was only using anaconda UI up to that point, didn't even switch to any 
other consoles. The only out-of-the-ordinary options I chose earlier 
were to modify the partition layout by hand & to use normal partitions 
instead of that whole LVM business - and the package options I mentioned 
above.

Where did it go wrong? Is there a fix?