Have you tried to resize the C drive, OS partition and run into problems with messages like Extend Failed?

One step that many of the sites that instruct on doing this is that you cannot resize the C partition as long as you have the D partition active or even if it is deleted, you have to remove it so that it shows as unallocated, then you will be able to extend the C drive partition without getting the Extend Failed messages.

It would be so helpful if Microsoft would provide some additional info on this.