Have you ever had the chance of migrating old memory to a newer hardware, only to find out that its not reading all those modules when they should be?
The reason for this is that, server hardware is channel specific, what I meant by this is that if your board supports more than one processor slot and who have no luxury of reading the setup manual. Chances are you are putting those memory modules on locations which they are not ought to be in. The hint is this. The closer the memory banks to a processor the most likely that that is the correct channel for those banks.