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You can run Classic mode ontop of OS X, but no direct booting into 9.
 
There have been cases in the past where firmware/system patches and/or extensions were made to allow older systems to boot up running upgrade cards and the like.

But this past performance provides zero indication of future support for any similar product for the new machines.

Even if someone did patch the firmware to seek out and boot from a OS 9 folder, there is no guarantee that OS 9 would remain functionally stable and/or everything would work.

The fan in the MDD is a prime example of problems that could crop up.

If the same problem crops up in a new MDD machine that you had patched to boot into OS 9, you'd either have to live with a full speed (or worse dead) fan until someone decides to write a program to monitor the CPU temp and control the speed of the fan.

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"There's always a possibility, it just requires someone to provide a product to fill the need -- but there's no guarantee that somebody will step up and do it."
 
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