I have the same problem. The issue may be the very large settlement imports. What I do, and it seems to work is: Step 1, save the game. Step 2, Exit to Fallout 4 main menu. Step 3, let fallout 4 run for awhile, check the disk access light it will be on for quite awhile. Step 4, when the light stops accessing the drive force the close with Task Manager. So far it has not corrupted any of my save files and seems to be the ONLY way to correct the problem.