Hi,
The Windows Search service is running, but i can't add C drive to the index.
1.Is there any error message when you failed to ad C drive to index? Were you using Admin account?
Kindly check the status message and relevant suggestions:
https://support.microsoft.com/en-us/help/4558579/troubleshoot-windows-search-performance
2.You may also go through the guidance on adding Locations for Search Index from below link and pay attention to the notes part as well:
https://www.tenforums.com/tutorials/58756-add-remove-search-index-locations-windows-10-a.html
Please note: Information posted in the given link is hosted by a third party. Microsoft does not guarantee the accuracy and effectiveness of information.
Thanks,
Jenny