MP 1,1-5,1 Mixing 1066 and 1333?

[l008com]

I have a Mac Pro, i believe it's a 3,1 that I flashed to 5,1.

It originally came with 3x2GB of 1066 RAM. After a while, I bought a pair of Crucial 1333 16 GB DIMMs and threw them in.

Great, now I'm running 32 GB, it shows up as two 16 GB chips, About this Mac shows it as 1333, and life is good.

But every once in a while, one of the chips doesn't show up. I get a red light on the logic board by the memory slots, and only one of the chips show. After labelling them and taking some notes, I can see that every single time it sees 16 GB, its always the same chip that goes missing, no matter which slot it and its brother are in.

This old Mac Pro doesn't really need 32 GB of RAM anyway, I could probably still run the original 6 pretty well. 16 should be plenty. BUUUUUT since I have the 6, I might as well throw it all in and get a total of 22 GB of RAM.

Ok here's the meat of the question. Every single time I try to boot with 1333 and 1066 mixed, the power light flashes and it won't boot. It boots fine with just the 1333 and fine with just the 1066. But any amount of mixing, and nada. Is this normal? I feel like normally faster ram gets "downclocked" to match slower RAM, of the same family. And on the one hand, given that I'm never going to use more than 16 GB anyway (most likely), I'm probably better off with 16 GB of 1333 vs 22 GB of 1066. But either way, I'd like the option

 

[tsialex]

I have a Mac Pro, i believe it's a 3,1 that I flashed to 5,1.

While you can cross-flash early-2009 Mac Pros (MacPro4,1) to MacPro5,1 firmware, you can't do the same with early-2008 Mac Pro (MacPro3,1), completely different hardware.

It originally came with 3x2GB of 1066 RAM. After a while, I bought a pair of Crucial 1333 16 GB DIMMs and threw them in.

While you can mix DIMMs of different frequencies, everything will work at the frequency of the slowest module installed, you can't mix different types of memories, like UDIMMs and RDIMMs.

Check if your original DIMMs are not un-bufferered/un-registered, Apple sent UDIMMs with MacPros back then. All DIMMs need to be registered or all unregistered with a MacPro, no mixing of UDIMMs and RDIMMs allowed. Your 16GB DIMMs are most probably RDIMMs and the original 2GB DIMMs are UDIMMs, not a valid config when installed at the same time.

Sometimes mixing DIMMs of different rank also cause problems, so try to avoid mixing 2Rx4 DIMMs with 4Rx4.

BTW do not forget ECC versus normal DIMMs.