First nvidia chipset for intel had a bug, bit31 of usb memory BAR was not decoded properly so usb could only use memory below 2GB. Nvidia's reference bios would always re-map any physical memory above 2GB to be above 4GB. If apple is trying to use ram above 2GB in the address range of 2GB-4GB and they are using the buggy version of the nvidia southbridge then you can get random hw hangs as the usb controller trashes memory. This is just one of the many serious hardware bugs in this chipset.
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First nvidia chipset for intel had a bug, bit31 of usb memory BAR was not decoded properly so usb could only use memory below 2GB. Nvidia's reference bios would always re-map any physical memory above 2GB to be above 4GB. If apple is trying to use ram above 2GB in the address range of 2GB-4GB and they are using the buggy version of the nvidia southbridge then you can get random hw hangs as the usb controller trashes memory. This is just one of the many serious hardware bugs in this chipset.