I have two 1GB DIMMs of DDR/400 installed on an Intel D875PBZ mobo. The board has four slots, and I have the DIMMs arranged in dual-channel mode. I have double-checked it and I have one DIMM in channel A, slot 0 and one DIMM in channel B, slot 0. About once a week, every few restarts, the machine reboots with the error saying that the RAM amounts have changed. When I check, it shows each DIMM as 512MB instead of 1GB. On occasion, it will show the correct amount, but it will be in single-channel mode only! Keep in mind, I swapped in these 1GB DIMMs recently from a pair of 512MB DIMMs. I tried updating the BIOS but have had no success at all.
Jeff, the Doctor believes that the RAM is not firmly seated in those slots or may have backed out of the slot over time or when the machine was bumped. The Doc has witnessed RAM that was not firmly seated playing havoc with a Core i7 machine. In that case, the PC had 6GB of DDR3/1333 installed, which showed up correctly in Windows and would even run, but would only operate in single-channel mode. The RAM was in the correct slots, but just not seated enough for the memory controller to run all three DIMMs in triple-channel mode. You likely have the same situation—a situation you probably could not reproduce if you tried. You should remove both DIMMs and inspect both slots for debris. If there is debris, use a can of compressed air to gently blow the dust or debris out. Then reinstall both DIMMs and make sure both are firmly seated and that the arms are locked in place.
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