My 8.2 machine ended up having dirty filesystems after rebooting becayse of bug in the network drive shutdown code in 8.2. When I tried to upgrade this system, 9.0RC2's installer couldnt' find the / filesystem to upgrade. I assume this is because it was dirty. The problem went away and I was able to upgrade on these filesystems after hacking out parts of the shutdown script.
The message given wasn't very informative. It was something like "unable to find / filesystem" or "unable to find mandrake linux installation" It would have been better to say - "all filesystems need repair before installation" (or better yet ask, fsck & repair them, but problably not time for that for 9.0).
