ReneWold, The issue you are seeing is Udimm specific. As the T310 only supports up to four UDIMMs. As seen on page 109. If you were to go to Rdimms instead, you would then be able to use 3 dimms per channel, allowing you all 6 dimms. Let me know if this helps answer your questions.
Good afternoon. I have a Dell PowerEdge T310 running Windows 2008 R2 server. It came with 2 x 2GB 1333Mhz Unbuffered memory sticks.(Total 4GB) I added 2 x 2GB 1333Mhz Unbuffered memory sticks and it worked fine giving me 8GB total memory. Last week I bought another 2 x 4G 1333Mhz Unbuffered memory sticks, to make it 16GB total, using all the 6 (six) slots available on the Intel XEON based motherboard. Now, the server will not start and gives me a memory error on the front screen. If I take out 2 sticks, using only the new 2 x 4GB and the 2 x 2GB memory stick the server will start with no problems. I am looking for answer on "why" it will not work with 4 x 2GB and 2 x 4GB sticks using all 6 (six) available slots.  I am wanting as much memory as possible and to utilize all the 6 slots. I can purchase 6 sticks of "registered" memory, but if that will not working either I am hesitating the purchase. Any help would be greatly appreciated. (Browsing Amazon, I do find a kit of registered models that they say will work being Dual Rank and having the ability to use all 6 slots:   
Dell Poweredge T 310 - Memory Problems