A Microsoft file hosting and synchronization service.
You can move your OneDrive folder to your D drive if it has adequate space and you intend not to eject the D drive. This should work fine using an external hard drive.
https://support.microsoft.com/en-us/office/change-the-location-of-your-onedrive-folder
I have tried using the Mlink command to link the folders I want to back up to identical folders in the one drive local folder, this has worked in part but never seems to totally sync all files to the cloud or track updates well.
From what I've heard this doesn't work well if at all.
I have also tried using AOMEI backupper to do this, while it works better in the sense all files make it to the cloud and its better at keeping on top of file changes, it needs to copy the files I want from their original location to the local one drive folder (mlink also does this) before it can back them up to the cloud.
Moving your OneDrive folder to your D drive will eliminate the need for making backups of your D drive files to your OneDrive folder on your C drive.