Something to check the next time you see the message is the size of your page file particularly if you have left it in its default location on your system drive and/or if you have set it to a fixed size. I would bet those D800 raw files use a lot of memory.
Am I correct in my understanding that you are first importing the files from your camera to the "E" drive and then copying the files to your
"I" drive when you import them into Lightroom?