Resizing C: requires "preOS" mode?
I have a brand new Ideapad laptop with 1TB HD (900GB in C, 20GB in a "lenovo drive" though not sure what this is, and roughly 80GB for a recovery drive). I want to resize C: down to about 400GB, then create a new F: drive to save my data at about 500GB. I right click C: in partition manager, then select "create partition". I put in the values, then hit "apply", then it tells me it requires "preOS" mode. I've googled around and heard lots of problems with this mode, such as failing to start up, so I just cancelled the operation for now. Can anyone tell me what this "preOS" mode is, and why it's required? I've searched around this site and nowhere does it mention preOS mode is required for a simple resizing task.