We have a Dell Precision T5500 Workstation. We have installed 72GB (9 x 8GB) with a riser.  Windows & the BIOS recognise all 9 modules for 72GB total, but only 56GB is deemed usable.  When we boot, we get a memory failure in DIMM 1.  Is there a BIOS or Windows setting we need to change?  The RAM is Kingston 8GB 1333MHz DDR3 ECC.

Any help greatly appreciated