Hello Art of Sound,
Thanks for your post on the Makie Onyx Drivers / Mountain Lion incompatibility. I am also in the same situation - love using my Onyx board/interface but out of luck as I have an older serial number. Here's a link to their page if anyone else wants to check their serial number:
http://www.mackie.com/products/onyxiseries/drivers/
I just spent some time on the phone with Mackie tech support and wanted to share this info with you guys:
The tech told me that even though it's not officially supported, they have an Onyx with the older 00 serial numbers working properly in Mountain Lion. However, I can't get mine to play or record without crackling/popping sounds. So I'm guessing someone replaced the chip on that one, I have no idea how they have the one magical one that works in Mountain Lion.
After reaching the conclucsion that I'm out of luck as many of us are with the older Onyx, he did say they sell the newer chip as a part but it would cost $190 plus installation at an authorized service center. So we concluded it would make more sense to buy a replacement Onyx with newer serial number (or another, better interface).
However, there's no way for us to know what serial number you'll get when you order. And the tech admitted to me that the newer ones are only a month or so old so most dealers will be selling the old (not Mountain Lion compatible) ones for a while.
Hope everyone can find another solution. Unfortunately, it seems like a lot of Mackie Onyx's will become paperweights.