Yes, Unbuffered won't work with the PC3L-10600r memory you already have installed. 
Hi there Was hoping someone would be able to advise me here.  Im trying to upgrade our RAM in a T620 we have.  Dual processor setup.  We currently have 32GB split across 8 sticks of the following RAM : [URL] I can clearly see on the specs of this RAM that its Registered.. These are in slots A1, A2, A3, A4, B1, B2, B3 and B4 I have been asked to upgrade the RAM by another 16GB and was sent 4 of these to do it : [URL]  I installed them in slots A5,A6,B5 and B6 respectively in a narrow window last night but the server wouldnt post - got the orange PST0129 error on the LCD display.  I didnt have time to remove all the original RAM and try with just the new ones to see if they were the problem, I just removed the new RAM and got the server back up and running. Im struggling to find specs on the new RAM that tells me if its registered or not but it does say Unbuffered - is that the same thing?  Is that the problem that I have a conflict with the RAM and need to go back to my manager and ask for different RAM or have I misunderstood the DIMM slot setup and put the additional RAM in the wrong slots? Any advice would be very helpful.. Thanks Jim
T620 RAM Upgrade help