I'm in the process of upgrading memory for my mid 2010 mac pro. I've purchased the following: Results for Apple Mac Pro DDR3 (Mid 2010) 2CPU Kingston's System Specific Memory 24GB Kit (3x8GB) - DDR3 1333MHz Part Number: KTA-MP1333DRK3/24G HTS: 8473.30.1140, ECCN: EAR99 When this memory is installed in slots 1, 2 and 3 and the mac start as expected. Hardware info displays it is running on 1066 Mhz (same as the previous memory), although the speed of the dimms are 1333Mhz. Now I want to use the memory that was in there before the upgrade, but every dimm I put in slot 4 I get the flashing led. Indicating the bad ram. Is there a best practice for this? I already followed the best practice of apple. But does not help.