Mac Pro and Mountain Lion, Memory Limitations?

Brand new MacPro 5,1 (mid-2012 I guess they call them, but weren't the 2010s also 5,1s?) - came naturally with Mountain Lion installed. This is a twin 2.66 Xeon 6-core.

Ordered up 128GB of RAM from OWC (came originally from Apple with 12GB, so 2GB DIMMs in slots 1-3, and 5-7). Removed Apple RAM and just installed 32GB in slot 1 and 5 - perfect.

Next, installed 32GB more in slots 2 and 6 - perfect. So we're running 64GB now.

Next 32GB - get boot chime, get gray screen, Apple logo appears, a few seconds later - screen scrambles. PRAM reset, no go, but figured I'd try it.

Long story short, it's not memory, or logic board, it's Mountain Lion. Booted off of an external drive with 10.7.5, bingo. 96GB. Install last 32GB and bingo again, 128GB - all DIMMs recognized and reading according to system profiler. Yes, Activity Monitor shows only 96GB wired as expected.

Contacted Apple, tech. stated the machine is only tested to and guaranteed to run 64GB of RAM, which it does on 10.8, and tech. was unaware of any pending "fix" for it, it's obviously a software issue. Tech. suggested that perhaps Mountain Lion was engineered to only use 64GB since that is how it was tested. Doesn't make sense for new MacPro, sheesh I'd hope any new machines were engineered to use >64GB of RAM!

Enough of my ranting - anybody here have any "magic tricks" to get ML running beyond 64GB? Am I ignorant of something obvious? Any help or suggestions greatly appreciated! Thanks.

Not much you can do about it.... here's what Mactracker says:

Maximum Memory 48 GB (Quad- and 6-Core Actual) 96 GB (12-Core Actual) 32 GB (Quad- and 6-Core Apple) 64 GB (12-Core Apple)

Memory Slots 4 (Quad- and 6-Core) or 8 (12-Core) - 240-pin PC3-10600E (1333 MHz) DDR3 ECC SDRAM (memory speed for certain configurations is 1066 MHz)

Why you were able to get 128 GB booting off an external drive with Lion, I don't know.

Why don't you give OWC a call on their toll free tech support line and talk with one of their techs. Those guys are usually sharper than the Apple techs. Let us know what he says.

Actually, it was OWC that suggested trying it with Lion to see what would happen! They were right - they've definitely run this configuration before. But the tech. was not aware of any specific fixes, Apple or otherwise, to 10.8 on this issue.

And you are right - the OWC techs. seem to be more on their game as compared to the Apple techs. - they must get to experiment more ;D and the Apple techs are most likely constrained by the "engineering", warranty issues, etc., too bad really.

Don't really need Mountain Lion, so we've wiped out Mountain Lion and installed Lion instead. Still, kind of disappointing to not be able to fully use the latest and "greatest" (?)......