Hi nadogmoney, The reason why those slots are disabled is that Advanced ECC mode combines two channels into one channel. As this server has three channels that leaves one channel as unusable. 
I found this in the owners manual: ftp://ftp.dell.com/Manuals/all-products/esuprt_ser_stor_net/esuprt_poweredge/poweredge-r420_Owner%27s%20Manual_en-us.pdf "Memory sockets A1, A4, B1, and B4 are disabled and do not supported Advanced ECC mode" Anyone know why this is?
R420 Advanced ECC disabled for memory sockets a1,a4,b1,b4 