Use recovery partition to add space to C drive in Windows 10.
- First go to the application recovery drive.
- Create a recovery drive
- At the end of the process select to delete recovery partition.
- The head over to this link to download a free partition tool.
- First delete the volume for the old recovery partition, example drive E.
- Then right click on the empty volume and select MERGE Partitions.
- Select C and E to merge.
- Apply then you will get prompted to reboot.
I just did this process of a Surface Pro with Windows 10 and was able to add about 7.5GB to my C drive.