first of all, take a full backup of everything.
if you've reclaimed the space that was being used by D, then it's easy to expand C.
1. On the server, go to Server Manager
2. Expand 'Storage'
3. Click on 'Disk Management'
4. Right-click on C and select select 'Extend Volume'
5. Specify the new size. (you can use all of the available space if you want)
6. That's it. Now reboot the server and you'll find the C drive is bigger.
This is the work plan we used answered by Eric C. When we formatted D: to free space, A RE-BOOT WAS NOT DONE. Is a re-boot necessary to do so C: can be expanded?
When we tried to expand C:, it was greyed out. This process takes several hours, I would like to be sure that if I proceed with this work plan, a re-boot is necessary after formatting D: creating free space to allow expanding of C: drive.
Otherwise we will likely need to use a 3rd party software such as Partition Magic.