One little loophole for those who want to upgrade an x100 to 9.1 and don't have a 9.1 CD - remove the HDD from the Mac and connect it to a newer machine, and run the OS 9.1 updater on the newer machine. Back when I had my 8100, I did this in order to get Mac OS 9.1 running on it - I don't have a retail OS 9.1 CD, just a 9.0 CD, so I installed Mac OS 9 on the 8100, then removed the HDD, put it in an external enclosure which I connected to my PowerBook 1400 (which supports the Mac OS 9.1 updater), ran the updater, and put the drive back in the 8100, and that install worked great for another 3 years until the logic board died.