I have a Mac Pro Mid-2012 6-core 3.46GHz, with 32GB (4x8GB) of OWC memory. I recently bought two 16GB OWC modules thinking I could simply replace two of the 8GB modules with the two 16s. It didn't work. Installing either new module with the 8GB modules wouldn't boot. I returned the memory thinking it must be bad. I bought the modules on Amazon; later I was reading the product page—I had missed a little notice that said you can't use those two 16GB modules with any other modules, only another of the same. The same goes with the already installed 8GB modules—they all have to be the same. To get 48GB using OWC you have to buy three 16GBs. Very disappointing since the four 8GB modules become useless in that case, or I can sell them back to OWC for a fraction of what they cost. However, I have another nearly identical Mac Pro that has 32GB (4x8GB) that isn't OWC memory. My question is, if anyone knows, will two Crucial 16GB modules work in conjunction with two of the installed non-OWC 8GB modules? What do other people do to upgrade their Mac Pro from 32GB to 48GB?