A case of bad memory
On the day after i picked up my Mac Pro from the University Village Apple Store, i promptly ordered two 2-GB DIMM modules from OWC (macsales.com) for it. The DIMM modules arrived the following week. After installing the DIMM modules, i swore System Profiler indicated that i had 6 GB of RAM. Several days later, i installed Leopard 10.5.2 update and on reboot, to my horror, i realized that the “6 GB” of RAM has become 4 GB. On closer inspection, the two 2-GB DIMM modules were being recognized as 1-GB DIMM modules!
Initially i thought the culprit was the 10.5.2 update. After getting some advice from AppleCare, i booted Mac Pro into maintenance mode and loaded Apple Hardware Test (AHT). AHT would freeze whenever the OWC DIMM modules were installed, regardless of the locations where they were installed (Mac Pro comes with two 4-slot banks of memory cards). If i took out the OWC DIMM modules, AHT would have no problem. Another user on Apple’s support forum had the same problem.
Good thing OWC has a pretty decent exchange/return policy. After contacting its tech support, i received a pre-paid UPS shipping label and now the “bad” DIMM modules are on their way back to OWC.