How to Resize the System Partition without using a separate drive:
https://www.youtube.com/watch?v=q1GP1qOReKA&feature=youtu.beThe separate drive method involves moving data to a secondary drive and then deleting the Data Partition. Resize the OS Partition and the recreate the Data partition and move back.
The method I showed above uses a boot-time partition editor to modify the partitions. Just make sure you do a full image backup before you do anything. Defragmenting the drive is a good idea as well as it makes it easier to move data across the drive.