exception: installation media already mounted

[Kenneth Porter <shiva sewingwitch com>]

I'm trying to install CentOS5 i386 from DVD ISO to my new second SATA 
drive. I boot from a USB key and tell it to use the DVD image on sda5 
(ext3, an FC5 boot partition), installing to a default partition setup on 
sdb. I tell it to install grub to sda (where my old FC5 grub install 
lives), with WinXP64 on sda1 as a non-default additional OS. Right after 
that I get an Anaconda exception that the installation media is already 
mounted. What now? What's the best way to debug this?