One other tip - if you are printing a large raft very slowly, you should double check that your communication settings allow enough time for that move to finish. S3D has a nice feature where it will automatically monitor USB prints and alert the user if the firmware goes MIA for long enough. Usually it's a safe bet that if your firmware hasn't responded to a command for 60 seconds, something is definitely wrong. So you can setup S3D to detect these problems and then it will try to automatically diagnose and solve them. It's saved several of my prints
But anyways, if you are printing a raft really slowly, you just need to make sure that the timeout value is long enough to where it is not falsely triggered by that slow move. So go to Tools > Firmware Configuration, select the Communication tab, and make sure that the "communication timeout" option at the bottom has a value around 30-60 seconds. If for some reason you don't want this feature enabled, you can also turn it off on that same page.