Couple of questions:
- Are you using the "Make Second Copy" option during Import, and if so which drive is the target?
- Where (i.e. which drive) is your catalog located? And which drive contains the OS and User files? IIRC, Lightroom likes to have 20% free space available, but I can't remember if that's the catalog drive or the temporary storage drive....it's just that only 468gb free on a 2tb drive is getting close to that 20% threshold.