Not even all the G4's could boot into OS9 directly, you just had classic support under OSX.

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I think the last gen MDD were the non OS 9.

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The Mirror Door Drive G4s spanned the boots-OS9/Doesn't-boot-OS9 transition.

If a MDD machine has a 800 Mbps Firewire port, then it will not boot OS9.

If a MDD machine lacks the 800 Mbps Firewire port then it will boot OS9.

However, getting the machines to run OS9 properly can be a challenge. There's a ROM file which needs to be in the system folder and if you don't have an original set of system disks it can be a pain getting things configured properly. If you do have an original set of disks, apparently it can still be a pain.

I have an MDD running Tiger 4.11 which will boot into OS9 but which won't run QuickTime for some reason. I can't remember if it crashes on boot if QT tries to load or if it just complains. I need to revisit it and fix things up.