My 2008 Mac Pro was limping along with the supplied 2GB of RAM, so I bought a couple more 1GB sticks. I know for sure that the sticks and the risers were pushed all the way in. But System Profiler still showed 2GB, instead of the 4GB installed. I downloaded the tech manual for the system and found that one of the original Apple-supplied sticks had gone bad.
So I bought another 2GB. This time around there were no LEDs lit on either riser, but the system still insists it has only 2GB installed! I put two modules on each riser (but I also tried putting all four on one riser), and they are the same brand as the ones supplied by Apple. The System Profiler report is only registering the module installed in the first slot of each riser—according to the tech manual, if the risers were bad, then all four LEDs would be solid red.
Are your memory modules the same brand or exactly the same part number as that on the Apple memory? We know that crucial.com stopped recommending certain part numbers, even within the same manufacturer’s range, because of problems they were having with reliability. The official Apple FB-DIMMs for the Mac Pro have a larger-than-normal JEDEC heatsink.
System Profiler has information about your RAM, under the Memory heading—including its part number.
So it’s just about possible that you have some more failed modules. Put just one set of 2GB in and see if it’s detected, then put the other 2GB in. This is just about thoroughness, though—I don’t actually think your RAM has failed. I’m not even completely convinced that the original Apple RAM had either. Missing RAM problems like this are nearly always caused by arguments between different modules on the risers. The only valid configuration for four DIMMs on a 2008 Mac Pro is to have two in the first two slots of each riser. But try swapping around which pair of DIMMs goes on which riser. It doesn’t seem like it should make any difference, but it’s surprising how often it does.
If this doesn’t help, you should try swapping the last DIMMs you bought for a different brand because you’ve evidently just run up against some subtle brand incompatibility.
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bsbeamer
January 18, 2011 at 11:42am
Buy brand new RAM from OWC (macsales.com) and then sell back all of your existing DIMMs, or return the 2 new DIMMs and sell back the 2 old/Apple DIMMs and buy all new RAM.
Having ANY sort of mismatch between brands, manufacturing, timing, etc has been EXTREMELY problematic on our MacPro's at work, so much so that when we upgrade RAM we do full upgrades and do not combine old and new.
