"Does anyone know of a way to force the unmount or some other way to continue the install?"
One possible problem might be automount and you disagreeing on what is supposed to be mounted. If you are using automount then try disabling automount by changing the line for your CD in /etc/fstab. Then reboot.
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Be prepared. Create a LifeBoat CD.
http://users.rcn.com/srstites/LifeBo...home.page.html
Steve Stites