Carla, either your hard drive was partitioned or a second hard drive was added. Either way, you probably have enough space left on your C drive to take care of Windows updates, etc. Of course, it couldn't hurt to move files to D: drive to save more room. Files on one drive aren't saved locally on your PC, they are on a cloud server.
Anything you save to your desktop is saved on C: drive. I keep files on D: drive, but save "shortcuts" to my desktop for quick access.
The "My Documents" folder is also going to reside on C: drive, but you can move/save those files to D: drive in another folder. I recommend a different folder name like "save" or "my files" though. To move, select all files that you want to move, right click, CUT, then go to the new folder, right click, PASTE. That will remove them from C drive, but will only completely delete them from C drive as long as the move was successful. If you get a message that the file name is already there, SKIP, and the file will remain on C: Drive until you can compare and delete on your own or rename it if necessary. Then you can cut/paste again if needed.
You don't have to worry about what is saved on One Drive right now, but if you want to move those files to your D: Drive, the CUT/PASTE should work just the same and remove them from the cloud.
Did you get your drivers updated?