Question Bootup alert message (Dell Precision T3500)

Aug 5, 2019
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I have had Precision T3500 for about 10 years. About 5 years ago, system properties began reading out total memory 8GB (I bought it with 12GB RAM), so I removed the defective module and left everything else untouched. Since then the machine gives the following message every time I boot it up (before "Starting Windows" comes up) : "Alert! Non-optimal memory population detected. To maximize performance, populate DIMMS as specified in the service manual. Press F1 key to continue, F2 key for setup utility, F5 key to run onboard diagnostics". When I hit F1 key, everything works just fine, and have been doing so for 5 years now. System info shows 8GB as it now should. Onboard diagnostics did not report any warning/error. I could just leave it like this and continue, but I am a bit curious as to what this means, and if I should do something about it, like, populate the empty slot with 4GB DDR3 module. Any suggestion? The machine has Win7 64-bit.

Thanks.