Sounds like you need "CD-ROM ToolKit" for the 3rd-party 'driver' for the Plextor drive... I have a Plextor UltraPlex 40Max and OS 9.1 would not recognize this CD drive unless I had CD-ROM ToolKit installed.
Was it possible that the driver existed on the older version of the OS?
If so, if you still have the old system folder hanging around somewhere, drag the extension into the new system folder.