I've got a 5,1 Mac Pro (2009 Firmware upgrade) which I've recently upgraded to a new CPU tray with 2 x 3.46 GHz 6-Core Intel Xeon. I've been successfully running 32GB (4x8GB) 1333 memory in it since I updated the firmware a few years ago. The memory has carried over on the upgrade and has continued to work fine. Now that I have 4 more slots, I thought it would be a good opportunity to ad another 32GB. I ordered another set of 4x8GB. After installing the the new DIMM's the Mac won't boot , and I get the blinking front light. I've removed the older DIMM's to test the machine with the just the 4 new DIMM's and it boots no problem. Tried removing one of the new DIMM's and replacing it with one of the older ones, and it doesn't boot. So, the Mac boots with the old RAM. The Mac boots with the new RAM. The Mac doesn't boot with a mix of the RAM. These are the original sticks I was using. Apple® — 974044A - 32GB (4x8GB) DDR3 ECC UDIMM PC3-10600 9-9-9-24 Apple ECC (974044A) And these are the new ones I just ordered. 32GB (4X8GB) MEMORY PC3-10600 1333MHZ 1.5V ECC REG DDR3 240 PIN DIMM 2RX4 The ones I just received seem to differ from the add. I actually received Kingston 8GB 2Rx4 PC3L - 10600R - 9-12-E2. Which appear to be a lower voltage (PC3L?) if I'm not mistaken. I'm not overly versed in memory so I'm not too sure if I should request a replacement or not. The add mentioned I may receive different manufactured sticks, but they would all meet the same specs. And Like I said, the machine boots with it, and does show ECC Enabled in the system profiler. Just wondering if I'm crazy, but these sticks should be compatible with each other, no? Is there anything else I can check, or test to see if I'm on the right path? Or could there be other issues I've created since upgrading my tray and CPU's?