Solution
Note: For purpose of this discussion, "volume" means a hard disk or hard disk partition.
If you plan to install both Mac OS X and Mac OS 9.1 on the same volume, you should install Mac OS 9.1 first.
This symptom occurs if you install Mac OS X on a volume that does not contain any version of Mac OS 9 and then follow with an attempt to install Mac OS 9.1 on the same volume.
To work around this issue, use the clean installation option in the Mac OS 9 Installer.
Related documents
58176, "Mac OS 8.5, Mac OS 9: Performing a Clean Installation"