A few weeks ago there was some discussion here and in ticket 54147 about the amount of system memory being calculated incorrectly (for at least some machines) when doing a UEFI boot, but no solution was determined.
If anyone has any ideas I can test them on the machine of ours that is exhibiting the issue for the next week or so before I have to put that machine into production (by switching to a bios boot if necessary). cheers mark
