Memory issues

[martva]

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.

 

[Umbongo]

The Kingston memory is Registered ECC. Mac Pros come with unbuffered ECC when you get them with 1GB, 2GB or 4GB DIMMs from Apple. You can't mix these two types.

 

[martva]

Hmmm. I did not know it was unbuffered... That explains a lot. Thx